All for-profit businesses can be viewed abstractly as “in the business of making money”, so this doesn’t really distinguish Jane Street in any way.

> … why/if what they do is useful to anyone?

The utility that Jane Street provides is to the be a persistent buyer and seller of equities. Basically you can call them at any time and buy shares or sell shares. Most shareholders do not trade very often so without a “market maker” like Jane Street it can be a lot of work finding a buyer/seller who is willing to trade on your schedule at the current market price. You’ll have to pay them extra to convince them to trade, which makes it harder to trade profitably. Jane Street significantly lowers the price and makes trading easier (“provides liquidity to the market”).