MAME is not a single emulator but a framework. As new games are added or driver bugs fixed, the version number increments. Each new version requires a corresponding matching set of ROMs.
Version Matching: ROMs are tied to specific MAME versions. Using ROMs from a different version (e.g., 0.230 or 0.250) with the 0.240 emulator may result in errors. MAME 0.240 Mame 0.240 Full Rom Set
Significant Bloat: A "Full" set includes a massive amount of unplayable files, non-functional game prototypes, mechanical fruit machines, and thousands of international clones (alternate region versions) that most casual gamers will never want to play. MAME 0