Is the /snap/core folder needed?

The /snap folder isn't a traditional folder full of files. So you don't really delete the contents of that folder and get space back (if that's what you're expecting). This folder is used when snaps are installed. If for example you installed the Spotify snap then you'd end up with a /var/lib/snapd/snaps/spotify_6.snap file which contains the compressed application and libraries. This gets mounted under /snap/spotify/6 and symlinked as /snap/spotify/current.

The files haven't been uncompressed to that directory, they're just mounted and read at runtime. You can see this with mount | grep spotify, like this:-

/var/lib/snapd/snaps/spotify_6.snap on /snap/spotify/6 type squashfs (ro,nodev,relatime)

The same goes for the core runtime snap, which lives in the same location, and is mounted in a similar way. By default on a clean install of Ubuntu there are no snaps installed, even core. However as soon as you install a snap, you get the core snap pulled in, which contains the runtime low level dependencies needed by all snaps.

While you could unmount the directory and delete the core snap, you'd break all install snaps doing this, and the system will make it hard for you to do that as a result.

Snap is package management tool which was Canonical's new package management tool. And this snap package management used by Ubuntu Software Center--I know this because I never use the snap tool but I have 13+ snap folders. That is the reason /snap folder exist, I guess.

And tool name is snapd. So when you want to install or uninstall package, you should use command like this:

sudo apt autoremove snapd

or

sudo apt autoremove --purge snapd

If you are installing software package with apt command or synaptic package manager, you don't need snapd package. So you can remove it.

Ubuntu Software Center will use 'snapd'--that's what happen to me. But after removed snapd, Software Center won't use snapd, it seems.

The instruction

sudo apt purge snapd ubuntu-core-launcher squashfs-tools

also gets rid of the useless /dev/loop mounts.