If you have to ask 'why would you want a locker', you havnt been in a situation where you could truly use one.
In serious mud as soon as one tire has less traction than the other an open diff with let you down, hard. Limited slip will let you down as soon as there real traction difference, such as one side being in a real mud hole and one tire is on traction.
If you ever exceed your flex limit (pick up one tire) at crawl speed you will need a locker because your limited slip likely wont do it, open diffs will very quickly leave you frustrated as the guys with lockers walk past you and make it look easy as pie. I know, I've done/witnessed both.
Having a locker is probably the single biggest offroad improvement I would recommend. Having said that, I still like my open diffs in my daily driver, some dudes say that they experience worse tire wear with a locker, some dudes claim no difference in tire wear. I couldn't tell you, cause my tires live a life of abuse regardless of the diff.