I only use ankleweights while conditioning, specifically "kneehighs", and while doing floor kicks, not while hitting targets.
I think it tones muscle and gives your muscles some endurance. And if you can go quickly in those things, you can go faster with them off.
While conditioning, I do a few things with them on. I lean against a t-bar and raise my leg up into side kick position and hold it, and other times just keep bringing it down and up slowly. Or, I extend front snap kicks slowly and hold them out.
I have 2 pound ones, 4 pound ones, and 6 pound ones. Depends what I'm in the mood for, but you should start out with a light weight depending on how developed your legs are and if you've used them before, and work your way up.
I do more with 2 pounds and less with 6 pounds.