My understanding is that they USUALLY sit before crawling and so sitting is seen as a developmental step. But crawling is not seen as a milestone because some babies never learn to crawl. All very confusing which is why I don't take much notice now.

In my DD's case, sitting wasn't a precursor to crawling though I suppose it depends what your definition of crawling is. My definition is getting from A to B without rolling which is what she was doing - kind of dragging herself along by her elbows but without really using her legs that much.