𝙒𝙖𝙛𝙛𝙡𝙚™

Most games run on game engines. Unity is one of such engines. Those engines get updated and devs sometimes upgrade their game to newer engine versions (sometimes quite annoying, sometimes even impossible). Usually a newer engine version comes with new features, optimisations, and bug fixes. Sometimes also with more issues and more bloat, so upgrading to a new version isn't always reasonable. Unity 6 does have some nice little features that, if used properly in the game, might squeeze out some additional FPS (don't expect a miracle). In particular, I might imagine that the devs would try to make use of the better gpu instancing/batch rendering, which could reduce the number of draw calls (less CPU overhead). And maybe some better occlusion culling (simply speaking, "not drawing stuff that's not visible to the player"). Maybe some better multithreading, but I doubt it. Hope that helps a bit.