Well rooting and custom roms use to make a huge difference on older phones like the Droid 1 because it would make it faster. The Bionic is already fast. That being said rooting gives you full control over the phone. You can theme it, modify it, freeze non un-installable apps, use root only apps (for screenshots, making app backups, etc), and you can also mess up your phone entirely, so a lot of research is needed before you root and install roms. Plus the encrypted bootloader makes everything harder when it comes to root on this phone, you can't go back if you install leaks/kernels and its kind of a pia to "unbrick" your phone. You have to tread lightly with root on this phone.
If you do root I recommend installing "safestrap" right away and making a backup of your system. Safestrap lets you get into a custom recovery by rebooting phone, if you don't have this it is much much harder to recover your phone if you brick (ruin) it.
* take a look at some of the Roms in the Roms section. It will give you an idea of what modifications can be made. Roms only work with certain builds for this phone, so read carefully. For instance I am on build 901 and I am rooted but no roms currently work with this Motorola build supposedly.
