With a resin figure it also gives the paint something to adhere to. I tried it once without primer and just touching the figure the paint rubs off easily. I recommend priming resin figures just like you would metal.
I am currently working on a Verlinden resin bust which I purposely did not prime as an experiment. I am using acrylics and am having no paint adhesion problems. So far, so good.
Definitely, use primer. Some say you don't need to prime resin figures, just wash them, but I tried it once. The paint didn't cover well & I wound up removing the paint which was a project in itself.
i come from an automotive spare parts background and i have tonnes of auto primer aerosols stocked up ..... i find its best to prime everything i do with it and so far to date i have lost none of the fine detail even though it appears to coat heavier than running regular primers through my airbrush