The weight limit exists because of Jeff Blatnick's (an Olympic gold medalist wrassler) influence on the Unified Ruleset. He took the NCAA and Olympic weight limits, modified 'em so that they'd all end in a 5 or 0 (to make it more accessible for the more-moronic fans to follow), and added the weight limit on heavyweight cuz' that's what they do in wrestling.
Wrestling used to not have a weight-limit, but they changed it in the 1980's because, while most of the time there were no issues (more on that later)-- in fact, prior to 1984, of all the NCAA heavyweight champions throughout the organization's tenure, there were only three guys who weighed more than 300 pounds (most of the champions weighed between 220 and 245. There would, however, occasionally be people who'd outweigh their counterparts by sometimes twice their weight, and it would show in the contest, which was perceived as an unfair advantage. So a limit was placed on heavyweight, which was understandable, and Blatnick used this same reasoning to justify its usage in shootfighting.
Unfortunately, though, boxing doesn't have a weight limit, kickboxing doesn't have a weight limit, submission grappling doesn't have a weight limit, Sambo doesn't have a weight limit (though they have a 250-lb weight class in some places, but not at the Worlds),
combat wrestling doesn't have one, Judo doesn't have one... so it's kind of ridiculous to impose wrestling-specific standards in a sport where other sports' disciplines and standards play as much a role in how a fight turns out as it does anywhere else.
The big thing is that, when you get above a certain weight-- I.E., ~220-- everything becomes a great equalizer. Each weight discrepancy poses its own pros and cons which vary depending on the skills of the participants involved.
Especially when striking's involved. That's why there've only been a handful of elite boxers who've weighed more than 300 pounds, why Semmy Schilt never weighed more than ~285, why Mike Tyson was dominating guys (and why a lot of the boxing heavyweight top-10 only weigh, like, 230), and why Fedor, Cro Cop and Nogueira could destroy so many much-larger opponents in their primes.
The UFC's influence on the creation of the unified rules is also a big factor. The image of a promotion's heavyweights can- and does weigh a lot [heh], and it changes a promotion's image when you have fighters who look like Zuluzinho or Butterbean, which hurts if that's not the image you're going for. So the 265-lb limit's enforced so stringently because they wanna protect their image, despite the fact that it's unfaithful to the essence of sportsmanship.
Fighters like Sylvia, Hunto, Thiago Santos, Prindle, etc. wouldn't exactly be better fighters if they didn't have to make 265.
Having- and not having to cut 20+ pounds makes a big difference. It may not be a fight-changing difference some of the time, but it's a big difference nonetheless. 20 pounds of water ain't no small thing, no matter your size. Sylvia in his prime could've potentially been much better in some of his fights if he didn't have to cut 20-30 pounds every fight. Bigfoot would definitely be better if he didn't have to, and possibly Lesnar, too. Schilt's K-1 career definitely would've hurt if he was cutting 20-30 pounds before the grand prix when his opponents were cutting nothing.