The best combo is probably draw-to-X engines. You can buy a bunch of Watchtowers, then as soon as one lines up with $4, you can buy Villa, go back and play the Watchtower, and repeat to draw your deck. Once you've run the Villas out, you have a semi-decent draw-to-X engine built. You certainly need a strong payload/attack that you can pick up early for this to be viable, though - just playing out a bunch of coppers every turn is not worth it.
The other main use is just avoiding terminal collision in slower money-ish games. Stuff like colliding Mountebanks that usually feels so bad now has an answer. You really don't want to overdo buying these in such decks, though - the top half of the card would be an underwhelming 3 and will often end up being a Copper.