I think the question should be, "when you don't want to use them".
1. Your driving across a hill with slick ground. Snow, loose dirt, mud. Your facing straight across but, now find yourself going sideways down hill.
2. You Need to turn sharp. When you turn, your wheels are diffrent speeds. Lockers make them turn at the same speed. Your wheels are turned and your still going straight.
3. When the route is actually harder then your skill level. All I can say here is, U-Tube.
I strongly suggest you drive with them off, I also suggest you stay in 2wd as much as possible. My reason is, learn how to navigate difficult terrain with as little help as possible. Learn how the vehicle feels when a tire starts to slip and practice throttle finesse.
What I have discovered over the years is, 4wd will get you into places 2wd wont. It will also get you more stuck if you don't have experience in that terrain. Lockers will get you places open diffs cant go. Each thing you add can get you deeper into trouble.
To directly answer your question, When a tire on the same axle has less traction then the other, power moves to the side with less traction. If it happens on front and rear axles, you stop moving and will spin tires (this is what I was talking about when I said Finesse).
Rocks. Big rocks, Ruts. If the ground is flat with moderately even traction, you don't need them. Ruts are another issue. One tire comes off the ground and all of the power on that axle goes to that tire.
With the rocks, Sometimes your going over with one tire. Axle twists and power on that axles shifts to the tire not on the rock.
Here's an example. I'm climbing a steep rocky hill. Lockers engaged. I need to turn while im on the hill, turn off front, turn and reengage front once im straight. If I needed to turn sharp, I may turn off both, turn sharp, reengage. Sometimes you may need to slip the rear and not the front. Now I disengage the rear only, slide the rear around then reengage.
What would be nice is if your lockers were completely selectable which I believe they are not. I don't believe you can select front only and I know some will only engage in low range. I will commonly engage the rear only in high range depending on terrain. Most people dont actually need them and I've seen people in areas they dont really have the skill for but got there because of lockers. U-Tube is full of people rolling their vehicles because they managed to get farther with lockers where if they didn't have them they would have been stopped due to lack of experiance.
