Howdy!
I'd say that method is a relic from times long ago. Nowadays the standards are so much higher, it doesn't make sense any more. For most of the parts it's best to paint them after glueing them to a subassembly - on a weathered model parts working close together can not have a totally different colour. And also you have to clean up practically every part, so if you absolutely have to pait the part separately, you clean it up and carefully choose a spot, to which you glue a holder, that will later be cut off. But the books from 30 years ago told you to paint on the sprue, and a tradition dies hard...
Hope it helps, have a nice day
Paweł