A fractional CMO is senior marketing leadership rented part-time. Typically $2,000–$8,000/month for a few days of strategy, vendor management, and accountability, versus $200k+ for the full-time version. They decide and direct; agencies and freelancers execute.
You might need one when: multiple vendors are rowing in different directions, you're spending real money ($10k+/month) with nobody owning the strategy, or every marketing decision still routes through you and you've become the bottleneck.
You don't need one to run one channel competently. That's what a good specialist plus your five-number scoreboard already covers. The fractional CMO buys orchestration; buying it before there's an orchestra is paying for a conductor to hum.