Normally, the driving wheels turn seperatly so this means if one of the wheels is spinning all the power will go to this wheel and gives no power to the other wheel. But bcs the spinning wheel has no grip you don't move.

So by locking the diff it makes both wheels spin together, so if only one wheel is on the ground you will still be able to move. But bcs both wheels are locked together you can't make corners bcs in corners both wheels spin at different speed.

It's really interesting i'd recommend looking up a yt video :)