I don't do drop sets that much but I have done them on pretty much everything... leg press, hack squat, chest press (barbell, smith and machine), bi-ceps curls, tri-cep extensions, calf raises etc etc. Drop sets are simply a useful way of exhausting the muscle and all it's fibers in a very short time period. Thinking back I tend to do them more when I am cutting but the answer why isn't scientific or smart. I could state it's because I am training more frequently so I keep my workouts shorter so use more drop, super and giant sets to accumulate more working sets in a shorter time period but it's not that because my volume tends to still be high (or even higher). It's more because I prefer just doing straight sets when growing as my main aim is lifting as heavy as possible and I don't want to dig into the cns too much at that time. So that leaves cutting periods were I basically just destroy myself in the gym for fun and I enjoy doing intensity techniques such as drop sets. When I am alone it makes sense to use pin loaded machines but when using plate loaded I just get off fast and drop the plates myself.