Huge can of worms! Quite a few well respected modellers are adamant that they never prime their kits. I often do but not always. I use a Halfords rattle can of their light grey plastic primer. I find a light colour base coat helps to highlight errors (visible seams etc) so I can correct them (or at least attempt to!!) before the top coat goes on.
For the “Kate” model I posted earlier I used the matching Alclad black primer which i gently sanded to make it nice and smooth before the aluminium top coat went on. For top coats that are yellow or white you get a much better finish with the right colour primer (some people use pink primer I think for a yellow top coat)
Gloss before decals helps to reduce silvering mainly and also a gloss finish makes washes work much better. Washes don’t tend to flow as well on a matt or satin finish.