I also regret not starting my playing with a metronome. Through long struggles I've finally gotten my timing to a place where its a LOT easier to play with someone else or even over a song I'm learning. There are many benefits to practicing with a metronome but most have already been discussed. I will go over just the basic major benefits though:

working on major timing issues such as macro managing your timing as well as micro managing.
What I mean by macro managing is practicing able to play in time with the down beats of every measure. This is very basic and therefore, VERY important.
As you can probably guess, micro managing is practicing the upbeats of every measure. you may have to set your metronome to 8th or even 16th notes to start off practicing this just to get used to practicing with it.
This will help bring your playing ultimately in time and very tight. This is all especially important if you ever plan on doing any recording at all, let alone a professional level.