I’ll have to disagree there. When you Favourite a post, the person that posted it gets a notification about the fact, while if you Bookmark something no notification is sent. In effect you are telling the person that you “Liked” their post.
Also, looking at the Explore section of Mastodon the following message is shown at the top of the feed:
These are posts from across the social web that are gaining traction today. Newer posts with more boosts and favorites are ranked higher.
So those Favourites are used by the algorithm to rank posts. Bookmarks are totally private and only used to save posts for your own use.