What's the use of non-double-acting baking powder?

I saw it in the store and I didn’t even realize it existed. What would you use it for?

Double-acting baking powder produces bubbles when it gets hot. Single acting only does so when it gets wet.

http://users.rcn.com/sue.interport/food/bakgsoda.html

So… if you want it to rise, but not a whole lot?

So whats the difference between single acting powder and just plain cream of tartar?

Cream of Tartar is just an acid (Tartaric Acid). Single Acting Baking Powder has an acid (cream of tartar) and a base (baking soda - sodium bicarbonate.) You need both an acid and a base in a recipie to get rise. (Unless you have compounds that create air when heated, such as the sodium aluminum sulfate stuff in Double Acting BP.)
Most modern recipies call for double acting (without mentioning it). So SABP would be useful if you had an older recipie (say, from the 1800’s or so).