They are 'market makers'. If you're a big {pension, real estate, rrsp}fund and want to make a trade, say a purchase, you need to call your trader directly. The trader will sell you some of their own (if they have it on their books), or will contact other people who they think may have extra supply on your behalf.

They are the 'grease of the economy'. In theory, it makes markets run smoother, which leads to greater efficiencies for everyone, and makes the world a better place.

(in theory)