You can use t-nuts for a lot of reasons. A t-nut specifically is for screwing machine screws into wood. This particular use case is using that machine screw as a leveler, but it's not the only case.

Sometimes you just want a large, solid bolt to hold things together, but allow them to be taken apart. In those cases I try to put the t-nut the other way around so assembly tightens the nut.