Yeah, it's initial peer discovery and connection setup (if behind NAT) I'm talking about. Once you've found the other PC and got a connection set up then all is well ;-)

Do these services also act as connectors in case of double NAT?

Which is a step backwards compared to bittorrent, since that can also provide decentralized hole punching, bootstrapping and peer discovery and not just the bulk data transfer. Plus being implemented at the TCP/UDP level allows more fine-grained control over the data flow, making it more friendly towards the network.

They're also needed for the actual javascript code that runs everything.

If privacy isn't your primary concern, and this is absolutely not private, you can also just use a redirection service and access .onion domains without installing Tor.

Just take any .onion domain and append it with .to and it should work. examplelettersblah.onion becomes examplelettersblah.onion.to and that works without needing to muck about with Tor.

This would be a horrible idea if you're concerned with privacy/anonymity. But, it'd make it easy for people to download your shared files.

That's exactly how Onionshare works. https://onionshare.org/