> The HFTs only work because the market would be made a moment later. A buyer and a seller already exist.

Not at all. If you, for whatever reason, need to sell right now, putting in a market order in an illiquid market can cost you a lot.

It can be discussed how big the contribution of HFTs to market making really is (large companies or exchanges often employ dedicated market makers, I believe?), but the benefit of market making itself should hopefully be obvious.