When you watch a video using Sync, like from redtube, you are actually asking the Sync server to find the video in the link and send you what it finds. Same with downloading a video. This is because the Dev constantly has to update the code that extracts videos from websites.

So he knows all the porn you watch.

(I'm just guessing what's going on based on my own usage of the app and seeing this url. Either that or it's pure spying and not needed at all.)

It's oauth token exchange for redgif stuff. Ideally it would be local, but that's not quite possible.

Apparently sync for Lemmy uses the same servers/domain name as sync for Reddit