1) It provides markets for securities that would otherwise by relatively illiquid.

2) It compresses the bid/ask spread so that the price better approximates what buyers and sellers are willing to pay.

The people who HFT affects are the institutions that are competing directly with them. For the most part it doesn't affect what you or I should choose to invest in as long as we assume we're investing, rather than speculating. (And even if we're speculating, our time horizon is probably much longer than a HFT firm's.