You don't HAVE to manually reinstall apps, your google account will download the apps automatically.
I'm no expert, but basically a ROM has many features, as well as the removal of (such as the "bloatware" apps included on your phone, a lot of ROMs do not have any of these apps included), that allow you to theme and customize your phone to your liking.
Most of the themes, if not all, on the forums require you to be rooted, and have specific ROMs which they are tailored to.
Rooting, which you need to be to flash a ROM, allows you to also underclock/overclock your phone. Underclocking saves battery while overclocking gives you better performance (while playing games and such), use caution when overclocking though.
There is so much more to flashing ROMs and being rooted as a whole, look around the forums trust me your gonna find a s*itload of information and your gonna find things out you didn't even kno you could do.
And don't be afraid to ask, the community has helped me so much in every step of the way!
Liberated Droid II
