Corrosion protection
There's a product called "Tinnit" that's available in a powder form like ammonium persulfate. It is made into a solution that you dip your freshly-etched board into to tin plate the copper. It's not cheap and if you start with an absolutely clean board and fresh solution, the results are really nice. But the solution precipitates out and doesn't seem to store well, so you better have a lot of tinning projects to make it worth the money. It's usually sold by companies that carry lots of PCB-making products.
To save money, I just clean all the flux off the back of my board after everything's been mounted and give the back three coats of Krylon clear spray. That's fast and cheap and works very well. It's not like a thick military conformal coating that's a bear to work through if you have to make a repair. It's so thin that you just resolder on a board as though it wasn't there and then just recoat the board when you're done or touch it up with clear finger nail polish.
Dean