Rooting means unlocking the OS so that you attain Superuser status. I.e. it allows you to findle with things you probably souldn't be fiddleing with, unless you really know what you doing. Most people do it so they can install other versions of the Android OS and to run apps that have functions that require the app get Superuser privileges to work.
Most of the time you would have to unlock the phone at the hardware level to do this (a process that is a lot more complicated and different on nearly every phone), but the Nexes phones come pre-unlocked. So it's just a matter of flipping a switch in a menu or installing a Superuser app.
In short unless you know you need it, don't bother. But if you do decide down the line (after reading up on the subject) to root/install another AOS/fiddle around, the Nexus phones are the path of least resistance. Also no crappy bloatware!