I see posts from time to time asking about lockers. People ask what they do, how they work, why you need them, and when to use them. Here is a real world example that is helpful for understanding.
Here is my Jeep flexed out. The front driver's side and the rear passenger tire have little weight on them. We will call those the "Spinning" tires. The front passenger and rear driver's tires are bearing most of the vehicle's weight. We will call those the "Stuck" tires. This is the place where the Jeep stops crawling forward in the real world with open diffs.
Open diffs always split torque evenly between the two wheels on the axle. Here, as I move forward the Spinning tire on each axle loses traction. As it loses traction, the amount of torque it can resist from the open diff essentially goes to 0 (it's not actually because there is some resistance from the tire itself being static and drivetrain losses, but I'm going to use 0 because it's easier).
Just like a torque wrench used on a free spinning bolt, the open diff can only output the torque that can be resisted by the tires. Remember, the open diff evenly splits torque. So, when the Spinning wheel can resist 0% of the available torque the Stuck wheel also gets 0% of the available torque, which isn't enough to drive it and the vehicle forward. The result here is that all 4 wheels get the same amount of torque, the Spinning wheels get enough to spin, but the Stuck wheels don't get enough to turn or drive the vehicle forward.
With a locker, the locked diffential forces each tire on the axle to rotate at the same speed. So, in this situation the locked differential would send 100% of the available torque to the Stuck tire and 0% of the available torque to the Spinning tire. Accordingly, the Stuck tires would get enough torque to drive themselves and move the vehicle forward.
Open diffs only hold you back when there are significant traction losses and those losses are different between each wheel on the same axle. This happens most often on large rock crawls and in deep mud.
Here, a little momentum could get me past the point where the Spinning wheels lose traction. Once the Spinning wheels have enough traction, the vehicle can drive forward even with the open diffs. So, another option would be to stack rocks or logs under the Spinning wheels to provide enough traction. Or, I could back up and go around. If I had lockers, I could use them to move forward from this point as well.
The downside to lockers is that they create bind when cornering (selectable lockers eliminate this problem when turned off) because the outside wheels travel in a longer arc and need to rotate faster than the inside wheels. This problem also is more of an issue where there is more traction. Additionally, lockers create more stress on the rest of the system (u joints, axle shafts, ring/pinion gears, etc.) because they will send all of the available torque to a stuck wheel until it either rotates or something snaps.
Long story is that lockers are great. But, you don't "need" them. You can go a long way without them in a Jeep. Understanding why and when they are a benefit will help you go farther without needing them. And, help you get the most out of them when you finally make the upgrade.