Buy slots makes sense for monitization. 99% of people will buy extra GTN slots and not favourite slots. I'm just hoping they aren't monitizing favourite slots now.
Favourite slots are quick links to something you like on the GTN. For instance, you might like a certain dye color but don't the credits. So you favourite it, and when you have the funds, you can instantly find it. It's a good way to keep tabs on hot selling items on the market.
Broadsword gave us 5 to start and I know people were asking for more.