When your bike has vertical dropouts and you want to convert it to a single-speed freewheel with one, two, or three chainrings up front, you need this. It wraps 20T, so you could, for example, have a 19t freewheel in back, and a 44x34x24 triple up front. Neat 3-speed!
The expensive alternative is the Paul Melvin.
The cheaper alternative is a regular derailer that, using minor bicyclery tricker, you can lock onto one cog of your cassette, and un-engage the shift lever.
The "Will Power" version, that works in theory but flops in practice, is to leave your geared bike as it is, shift to one gear and keep it there.
Don't use this or the Paul Melvin or any other chain tensioner on a fixed cog (no coasting). The wheel will lock up dangerously. Only on freewheels.