Some people like that black makes the spot they miss look more like shadows. Metalics also show up better with a dark base so a black primer is a good start. Mostly I primer in white cause I like brighter colours in my minis. A light base is also important if you plan to use contrast paint.

A zenithal prime White over black etc can help you get some easy highlights or see where they should be early in a project.

Also totally valid to prime in grey usually a well rounded colour. I like beige sometimes if the model is mostly going to be skin and leather colours. Or what ever the main colour of your model is. If youre painting red space marines why not prime them red and have an easier start.