It gives the ability to get across or up something with much more control.
I just bought my JK about a month ago. It was already geared and has all the "essentials", minus lockers. I've always had lockers in my other 3 Jeeps, so I was bit concerned when I took it for it's first trip last weekend.
Granted, we did some what I would call light to moderate trails, and did just fine. I would have preferred doing some other trails that would rate a bit more difficult, but the limiting factor was the unknown as I/we haven't seen any of the trails that we were doing before.
Typically, when locked I wouldn't have given it a second thought... just give it a shot and know that if I can't do it or pull cable while in the process, it simply can't be done by me for that day. The other two rigs I was with were locked, so I would send them out first and they'd radio back their opinion before I headed up.
We worked it out, but it was a bit of an inconvenience The lockers add some piece of mine of knowing that you've got all the available traction you're going to have from your rig, that day. You won't be left wondering if you could have done it... had you been locked.
PLUS the added value of control... less parts tend to break when you can control where/how the power is applied.