Because sometimes we want full manual control. And being able to hold the shutter open then quickly close it when we need to is really useful in some situations. For example taking a long night exposure, but want to close the shutter when you hear a car approaching with headlights that could ruin the shot. Better than setting say 43 seconds and hoping for the best. When dealing with long exposures it's better to have more control than lots of ruining shots based on guessing an exposure.
I've found that's true with astronomy photography. And a remote trigger is something you should use with a long exposure anyway - to avoid moving the camera at all.
Intervalometer for Axe, one of my fave accessories, it goes with me whenever I have a camera with me(and that is everywhere).
I find bulb mode handy for light painting, set to bulb cover lens with black card get mate into shot with sparklers/torch etc remove card, do light show then put card back on before mate walks out of shot, you get a nice clean light painting that way.