We have warned you about cryptojacking scripts on websites and in apps: they use your device to mine cryptocurrencies. We have warned you about malicious ads that are linked to all kinds of cyber threats.
And now guess what? correct: ads have been caught for stealth mining. Hackers infect third-party ads with pieces of code that are capable of mining the Monero currency even on devices with relatively weak CPUs, like smartphones.
Ad networks and publishers are more often than not unaware of this filling inside their ads. Which might be a reason to sympathize with them and send them some mental support, but that is no reason not to use an ad blocker.