Rooting allows the user to unlock areas of the device's system and permission level that the OEM says you shouldn't have access to.
This allows you to run custom ROMS (operating system), remove or truly disable bloatware, have the option to do full system back ups or individual file backups and must importantly the data and settings associated with them. These are the most common things people look for when rooting their device.
Only real big disadvantage which isn't a concern to most is your device in its rooted state it's security level is compromised if in the wrong hands. But there are still levels of security the can be used to protect your device.
Sent from my Galaxy Nexus using Android Central Forums