A Target ROAS Isn't a Bid. It's a Ceiling You Name for Yourself.
Automated bidding didn't remove the auction. It replaced the open market with a form that asks every advertiser what they're willing to pay, then rations demand along the lines each one wrote down.
The setting reads like a bid. It behaves like a rationing form
Bidding with no ceiling lets advertisers chasing the same audience push each other's costs up without limit. A target ROAS or target CPA exists to stop that, from the platform's side: instead of an open auction setting the price, every advertiser declares in advance what they'd be happy paying, and the available demand gets allocated along those self-declared lines.
The same number means something different in every account
A target ROAS can't be judged on its own, because its effect depends entirely on average order value, and that varies by account. Set high against a low-ticket product, it throttles cost per acquisition until volume disappears. Set at the same level against a higher-ticket product, it barely changes anything. Two advertisers can type in the identical target and get two completely different outcomes, and nothing in the dashboard explains why.
A ceiling can't be audited from inside the room it defines
Once a target is set, everything the platform reports back, cost, conversion count, ROAS itself, is calculated against the pool of auctions that ceiling made eligible in the first place. Moving the target doesn't just change performance. It changes which auctions the account competes in at all, and the platform has no way to show what got left out. The only way to know whether a target change cost real customers is a record of orders that exists independent of the system doing the rationing.