What are the benefits of a custom ROM anyway?
Depending on the ROM, it varies from small tweaks (changing pixel density to give impression of higher resolution etc. - can be done on any ROM), preinstalled scripts (apps2sd, better memory management - these things can be installed on any ROM, they just often come preinstalled in custom ROMs), removed bloatware, up to custom kernels (overclocking CPU etc.) and security precautions (fixing bugs & vulnerabilities in Android, allowing the denial of individual permissions on apps), or tweaking preinstalled apps (add extra options, change the look of apps). Most custom ROMs will be "themed" a bit different to stock as well. Often they'll come with certain apps preinstalled as well (either replacements or additions to the ones normally there).
EDIT: It is worth noting that many of these things CAN be done without installing a custom ROM. If you have root, you can tweak your stock ROM a lot.