Just referring to the blow molded cases here--not bags, Systainers, etc.

Use em? Keep em for when you sell the tool? Toss em?

At first I kept the tool in them. They were a hassle because they took up a ton of room under my bench and I'd end up with stacks of them sitting out while I used the tool. Any more, for all my shop tools (except Festool, see below), I put them in storage. The tools get stored in cabinets. When I've sold the tool, 1/2 the time I forget I've got them. Eventually I throw them away anyway.

I do use a couple--I keep one for my 18V drill out for when we go camping. I put a socket adapter in the drill and use it to run the stabilizer jacks up and down. And I keep my hammer drill in one because I use it more out of the shop than in the shop and there's room for my masonry bits.