For one thing, it allows you to remove/uninstall the bloatware/trial apps that Verizon has preinstalled on the phone (i.e. Blockbuster, Lets Golf, Rockband, etc...) either manually or using apps that have been written to do so. You'd also be able to backup your app data and restore from these backups in the case that you need to switch phones and such.
And if you want to take it a step further, you can install/backup entire roms that have been optimized or customized to fit the look and feel you like.
Basically, it gives you full access to the phone, which allows you to customize it to your liking. Obviously, it does also allow you to really mess up your phone, but I think as long as you read and follow instructions, you'll be ok.