They exist as a way of pulling people towards issues and as a release valve if the primary party gets too corrupt.

Take the Green Party in Canada. Without it, the remaining left(ish) parties in Canada only have to go a little better than the Conservatives on the environment to get the environmental vote. With the Greens in, they have to at least meet them half-way or they risk further splitting the centre-left.

They also sometimes hold the balance of power. This puts their issue front and centre.