Let's say you're a startup trying to advertise on Google, and somebody has paid a bot network to fraudulently click on your ads. Now Google can't detect that those clicks are fraudulent, so you're billed for them.

Your channel efficiency unavoidably goes down, which increases your cost of customer acquisition because your other channels cannot pick up all of the slack.

Increasing the cost of customer acquisition is going to be bad for your business. You will either need to reduce costs (by hiring less, for example), or increase your prices.