[hexagonal]

It creates liquidity, which makes it easier to buy and sell shares when you want to buy and sell them.

Unfortunately, the only time you really want liquidity is during a crisis, which is when all the HFT firms exit the market, since they're in it to make money, of course, not to act as a regulated and guaranteed market maker.

[theorique]

Regulated and guaranteed market makers are both (1) required to post a bid and ask even in crisis situations (2) high-frequency and high volume traders because they need to deal with a great amount of market order flow. These things are not necessarily exclusive.