What is the purpose of snapd other than to download packages from external sources?

It is a package manager. You can use it to install a .snap file (snap package) that you have downloaded by some other means, or that you have built yourself locally:

From https://docs.snapcraft.io/build-snaps/your-first-snap#install-the-snap :

Now that everything needed for the hello app to run has been packaged inside a snap. You can install it locally.
The --dangerous flag needs to be used here because your snap has not been signed.
$ sudo snap install hello_2.10_*.snap --dangerous
hello 2.10 installed
Congratulations, you’ve just made and installed your first snap!

Hello,

The problem is not that you can install non-free packages or add non-free repos. So can you in apt. The problem is that snap is using Snapcraft repository by default. Just try snap find skype, snap find clion or snap find spotify.

The problem tracked in this issue is that you can install non-free packages or add non-free repos - including skype and others.
Which packages can most easily be installed due to the default configuration of the tool is a subset of this general issue.

I've been informed that it is at least *theoretically* possible to point snapd to a only free "store" https://forum.snapcraft.io/t/external-repositories/1760/7
This might help us not have to remove snapd and then patch and maintain all the software that depends on snap and snapd.