Having the hammer means your team can throw the last stone in an end, making it much easier to score at least a point and win the end. After a team scores, the hammer will switch to the other team to even the playing field.

In professional championship curling, it is decided by who can throw a rock closest to the middle before the game starts. In general recreation curling, it is often decided by a simple coin flip or rock paper scissors.

More Info: worldcurling.org