Most funds are not assessing whether a film should exist. They are assessing whether this team can deliver it. A producer attachment is the fastest way to answer that question.
Funding bodies rarely reject a project because they dislike the story. They reject it because they cannot see, from the material in front of them, how it gets made. A producer attachment is the shortest route to that answer.
An attached producer supplies delivery history, a company with accounts, and a track record within a budget band. That converts an assessment of taste into an assessment of capacity — which is a question with a factual answer.
This is also why attachments are sequencing decisions rather than favours. Approaching a producer before the project can support the conversation wastes the one introduction you get. Approaching after the material is ready makes the same conversation short.
In practice, the order is: establish what the project can credibly claim, define the producer profile the project needs, then approach a small number of companies for whom the film is a plausible next title.