Everyone I fished with back in the 60s used them. We used to go on 3 day fishing trips all over the place and each of us had one rod and reel. Snap swivels were essential then. I think the popularity of Texas rigging a plastic worm was a reason people got away from them. And with tournament fishing folks started keeping multiple rigs ready, and there was less need to make quick changes.

A lot of baits work better with a snap swivel; it gives a Johnson spoon a lot better wiggle. I also still use one with an inline spinner like a snagless Sally.