Bitcoin (and altcoin) mining is starting to use a lot of power (e.g. https://www.theguardian.com/technology/2017/nov/27/bitcoin-mining-consumes-electricity-ireland). Besides keeping the Bitcoin ecosystem alive, spending that power has no other practical use.
Some companies are starting to combine mining with other useful features, like heating.
https://www.qarnot.com/crypto-heater_qc1/
I agree it's a complete waste of electricity to produce all that heat and do nothing with it - even worse you have to spend more electricity to vent that wasted energy!
And some of those companies with a lot of mining servers get cought because they use elctricity illegal
