What is the role of purified linseed oil in oil painting?

It makes the paint flow better. It’s the equivalent of using water with watercolors or acrylics.

But here’s a tip. Well, two tips. One is, you’ll get a faster-drying thinner if you mix your nice linseed oil with some turpentine. The other is, if you do this, you will get what we call a “lean” mixture of paint and medium (the mixing substance is called “medium”). It’ll be “lean” compared to “fat” paint – plain paint out of the tube, unmixed with anything that makes it more liquid.
So the tip is this: when painting, you never want to paint lean over fat. You want to paint fat over lean. The reason is that the lean layers will dry faster, being thinner. If a layer of thinned paint dries on top of a layer of unthinned paint, it will be completely dry and cohered to itself, making a skin, before the fatter layer under it has had a chance to
dry. So when that does dry completely, it will condense into a smaller volume under the already-dry upper layer, which will then
crrrrr a ck.
You’ve seen this.
It’s okay if you want it to happen. But if you don’t, then remind yourself: fat over lean.

phew that was fun

First two responses were great, plus oil painting takes forever to dry if you don’t use it!!

Also, if you are learning to paint on your own- and making a representational painting, remember to put the shadows in first. Sometimes I make an underpainting in Acrylic paint first (sometimes with just black and white, sometimes simple colors)...and (when dry of course) then go over it with oils. I never learned that in school, but heard it someplace, and it has really worked for me!

It’s part of one’s medium.