I never really used one. We used the jelly roll and a cover and the cover held it in place well. We would trifold a lot, too. I had a lot of covers so I didn't mind if we got poop on the cover. I wasn't confident enough to put a snappi on a wiggly crying newborn. Now that I am more experienced with cloth we might use a snappi more but most likely not.
I used one with my first child, and second, and will with this one. Since newborns are kinda scrunchy, I like being able to secure the diaper and then get the cover on.
I never did. I used the angel wing fold and the cover held it all together. I never had a regular poop get on the cover, only ones that would have been major blowouts anyway if not for the cover. I only bought my first snappi 2 months ago when I bought bigger prefolds since I loved my infant ones so much. Now I find she's too mobile, the prefold scrunches inside the cover without one.
I think you do. They're cheap and nice to have around. My DD had explosive poops, and she was very skinny, the snappy helped contain it to at least the cover.
Re: Prefold question
No, but it helps.
Fastening the folds helps contain all the newborn poop. But you could use pins, or other closure devices.
Or you could just trifold and accept that you'll go through more covers with the poop getting on them.