J.T. Realmuto is probably baseball's best catcher, which says a lot about him and a little about the relative weakness of the catching position in today's game. (By one value measure, he's the least-imposing "best catcher in baseball" since Mickey Tettleton held the unofficial crown in 1989.)
Still, it's easy to see why the Marlins placed such a high value on Realmuto's services this offseason, before finalizing a trade on Thursday that sent the catcher to the Phillies. It's difficult to find good catching, and he's a fantastic one, in his prime, reasonably priced for the next two years. If Realmuto merely repeats his 2018 line of .277/.340/.484 (139 OPS+) -- basically Matt Chapman's bat wielded by a good defensive catcher -- he'd have been a strong addition to nearly any team.
But what if there's more in there? What if by simply leaving Miami for Philadelphia, there's a possibility that Realmuto's bat could get an additional boost?

No, we're not talking about escaping a lack of lineup protection from a weak Marlins lineup, which is more myth than reality. We're talking about getting out of Marlins Park, which played like an extreme offense-suppressing stadium in 2018. We're talking about getting past some of the most serious home/road splits we've seen in years.
This isn't hyperbole. Look at the difference between "Miami Realmuto" and "road Realmuto" over his career:
Realmuto at home, career
.244/.292/.384, .291 wOBA, .279 BABIP
A .291 wOBA is basically what light-hitting up-the-middle players like Amed Rosario and Manuel Margot put up in 2018.
Realmuto on the road, career
.310/.358/.494, .364 wOBA, .356 BABIP
A .356 wOBA is basically what powerful sluggers like Javier Báez and Khris Davis put up in 2018.
That's a 73-point difference between Realmuto's home and road marks, and if you're wondering if that's a big deal, it is. As FanGraphs' Jeff Sullivan noted in December, it's the largest "home-field disadvantage" since 2002 of any player with at least 1,000 plate appearances both at home and on the road. It's worth noting that among the top eight names on that list, two logged time for the Marlins in recent years: Derek Dietrich and Logan Morrison.

Even if we're just looking at last season, Realmuto's best year to date, the differences remained.
Realmuto at home, 2018
.269/.329/.444, .332 wOBA, .313 BABIP
Realmuto on the road, 2018
.283/.350/.520, .372 wOBA, .311 BABIP
Ignore the relatively small difference in batting average, because that's not really the point here. Look at the large differences in on-base and slugging percentages. Realmuto's great year came in a park that kills offense. That much seems certain.
But it can't just be about Realmuto, can it? That is, if Marlins Park hurts hitting to such an extent, we need to see it in more than just one player. One way to do that is to look at hard-hit rate, or percentage of batted balls hit with exit velocities of at least 95 mph. If you sort that just by ballpark, you're obviously influenced by the talent level of the rosters who play there. For example, the parks with the two highest hard-hit rates in 2018 were in Anaheim and Boston, but that may say more about Mike Trout, Mookie Betts and J.D. Martinez than it does about the venue.
One way around that is to look at the hard-hit rate from visiting players in each venue. This could of course be affected by pitching talent -- good pitching should keep hard-hit rates low -- except that in this case, Marlins pitching wasn't good. Miami had a 4.76 ERA, highest in the National League, and it allowed a .424 slugging, second-highest. Our default expectation here should be that a poor pitching staff allowed a ton of crushed baseballs.
However, that's not what happened. Not at all. If we look at the 30 ballparks, ranked by highest hard-hit rate allowed by home pitchers, we'll see that Marlins Park came in last. No ballpark saw a lower hard-hit rate by visiting hitters.

There's a million ways to look at this, really. Visiting hitters had the lowest average distance (277 feet) on fly balls and line drives at Marlins Park, a full 21 feet behind Coors Field (298 feet). Visiting hitters at Marlins Park underperformed their Expected wOBA by 58 points, second-largest gap in the game, a full 161 points behind what happened in Colorado, where hitters overperformed by 103 points. Remember, this is all off a pitching staff that generally wasn't effective at limiting hard contact.
The reason why that happened is a little hard to suss out. It could be something with the batter's eye, or the atmospheric conditions inside the park, or the way the baseballs are stored there. It could be a combination of several of those factors, or something else we haven't considered. Whatever the root cause, it's clear that something about Marlins Park is making it more difficult to hit there.

So, in theory, this makes Realmuto even more appealing. The idea of getting him out of Marlins Park and placing him in Citizens Bank Park means that you'd expect his overall line to improve. After all, Philadelphia was the fourth-most friendly park for home runs in 2018, while Miami was 30th. Right?
The answer, of course, is "probably." You can certainly make the case that the move from Miami to Milwaukee boosted reigning NL MVP Award winner Christian Yelich in his first year as a Brewer in 2018. Leaving Miami didn't do the same for Marcell Ozuna, though his first year in St. Louis was marred by a shoulder injury so it's difficult to know what effect the ballpark change really had. Giancarlo Stanton's move from Miami to Yankee Stadium resulted in fewer home runs, but it wasn't reasonable to expect another 59-homer season, regardless of the home park.
Then again, the issue in Miami isn't spread to all fields. There's ample evidence that the biggest power suppressor in Marlins Park is trying to hit to right field, which might explain why leaving town affected the lefty Yelich more than than righties Stanton and Ozuna. Realmuto, of course, hits from the right side, but the effect on his power has been clear. Let's repurpose a chart Sullivan used to show just how hitting to the pull side at home was fine for Realmuto, but how hitting up the middle or to right field really hurt him.
Realmuto wOBA, career
Pull -- .410 home, .415 road (.005 better on road)
Center -- .298 home, .459 road (.161 better on road)
Opposite -- .211 home, .375 road (.164 better on road)
Those are pretty significant differences. We know something is going on in Miami, but we don't know what. We know Realmuto had a strong year there regardless. We can be reasonably certain that getting out of there to any other home park would have helped -- especially the hitter-friendly home park in Philadelphia.