You don't always have to. Just know it's there and you'll know when it'll be useful.

The page already explained it. Sometimes, animators like to loop several things, but don't want them to start the loop at the same time 'cause that looks unnatural for a lot of subjects. So they make some things start a loop a little later than others, while keeping the loop duration the same. That's what this is for. Key Offset is just a loop-friendly Key Shift. I wonder if they should just merge this into one thing.

The uses are innumerable. hair. anything that flaps in the wind. different legs. maybe bits in a fire animation. If you don't know what Overlapping Action is, you should look that up.