People like playing drums too, but a drum machine means that if you're not any good at it or too busy but you need drum sounds you can have drum sounds.

There are rights issues if the result is it replaces a particular singer, if you made it so that Sneaker Pimps can fire Kelli but still have her voice on subsequent songs that's a problem. But suppose you're a bedroom musician, and you realise you've got a piece that really wants somebody with a different voice than yours to make it work - you can pay someone, but technology like this offers a cheaper, easier option.