Miners use specialised hardware (ASIC - application-specific integrated circuit) to mine. They are only useful to calculate hashs efficiently.

is there no way to make 'good' use of them? whoever comes up with an idea might become a billionair.

Proof of work is intentionally useless to the miner to increase the cost of attacking the network. There are initiatives like curecoin and foldingcoin which integrate folding@home into their proof of work, but the output of that is still "useless" to the miner and relies on the folding@home team not manipulating the process, which becomes a bigger risk once hard rewards for folding are introduced.