Since I changed over to Acrylics I have found that I no longer have a choice concerning primer
I used it when I used Enamel paints, and kept the habit when I made the switch,,,,,,,which is a good thing, because some Acrylics won't stick to plastic unless there is a primer or at the very least, a coat of some other brand of Acrylics on the plastic
I think it might be different for the people using the two Japanese Acrlyics*,,,,,,,,but, for the rest, we either wash and prime,,,,,,or watch our paint practically sheet off at the merest glance
I only mention Acrylics because some people might read this and "need to know" about those,,,,,,,,,,the two best primers I have found for those is the LifeColor white primer, and the Vallejo Urethane,,,,,,,although Future can work as a primer when in a pinch, just add a touch of paint or gauche color(acrylic) if you want to see your progress when you spray it,,,,,,after all, lots of people put paint over one coat of cured Future, that sticks okay, then it is sealed
Rex
(*I can't use Tamiya or Gunze here at our house)