1. Not enough conversions
Automated bidding learns from conversion volume. A handful a month is not a training set, it is noise, and the bidding will chase it.
2. Tracking you do not trust yet
Smart Bidding optimises toward whatever you told it is a conversion. Point it at double-counted purchases and it will confidently buy more of the traffic that produced them.
3. Deliberately buying data
Sometimes you want coverage on a specific set of terms to learn whether they convert at all. Manual control gets you that answer faster and cheaper than waiting for an algorithm to explore.
The migration path
Manual or enhanced CPC while volume builds, then maximise conversions, then target ROAS once there is enough history to set a target that means something.