A straddle is used in some cash games to promote some action.

There is a SB in, BB in, and a straddle in. The straddle is double the BB. It puts more money in the pot and usually puts more action in the game because 3 people not 2 already have money invested.

Usually it is agreed to by all players and the straddler has the option whether or not to do it.

Why would people do it? It gives them position pre-flop and if they pick up a big hand there usually getting paid off in a big way.

Note about the straddle though, that if the original straddler is re-raised he loses his option to re-raise when it is his turn. But if there is a raise and re-raise in front the straddler may raise, call or fold as he pleases.