Snapshots are the weekly (at least they used to be, I think they're less frequent now) development snapshots - they are explicitly pre-beta versions that you have to opt in to.

They are explicitly a beta channel, and in fact if you create a world on a snapshot you don't have an upgrade path to future releases (unless you start hacking at your world files with an NBT editor). So most of the community ignores them and waits till release.

Snapshots do let you upgrade worlds to future releases and you don't need to do any NBT editing, just upgrade the game and load the world.

Huh, I guess I assumed that based on the behavior of most of the experimental flags offered on world creation. Has that always been the case?

I think mojang is taking a turn away from experimental feature flags these days, so I guess this situation will get better over time.

Though there are of course no guarantees there won’t be problems with the world.

They're previews of what they're working on and what they're going to add to the next version. You don't get auto updated to them, you have to select them in the launcher.

As you would expect from the word "snapshot" contrasted up against the word "release", might I add.

Snapshots are only for testing. They are sometimes not forwards-compatible.

they're not useful to users looking for the latest features. They're useful to mod developers who'd like to see how internal changes might affect their work. For example.

(eh, most people I know occasionally touch the snapshots for new features. Way back when the nether was updated, nobody I knew was on the release version because the nether was simply so much better that it was hard to play the older versions)

Minecraft snapshots are beta. A default installation of the Minecraft launcher will not download and install snapshots.

You have to opt-in to it.