CPM can drive your costs down on a highly targeted niche. Let's say you want to target a female tech journalist living in SF and using Slack, you'd rather use CPM than CPC as you're pretty sure there are only 20 people that match the criteria and even though you'd pay 50 USD per 1000 impressions, you'd only pay 5 dollars to ensure each person in this particular niche sees your ad 5 times!