In a great many firms the proposal is produced under time pressure by the person who ran the call, using the last one as a template, and sent with an apology for the delay. It restates what was discussed, prices it, and asks for a signature.
That document is doing almost none of the work it could do. By the time it arrives, the buyer has usually made a provisional decision and is looking for a reason to confirm it or to stall. What they get is a summary of a conversation they were present for.
What it is for
A proposal is the artefact that gets read by the people who were not in the room. In any purchase above a certain size there are such people, and they are deciding on the document rather than on the conversation. Writing it for the person you spoke to is writing it for the wrong reader.
It has to state the scope, the fee, the limitations and the exit path. Limitations are the clause most often omitted and the one that does the most for credibility, because a document that describes what the engagement will not do is visibly not a sales document.
Why it is never agreed on a call
A scope agreed verbally is agreed at the resolution of speech, which is lower than the resolution at which the work will be delivered. Both parties leave the call believing they agreed, and the disagreement surfaces in week three when it is expensive.
Writing it down before commitment also removes the pressure that a call creates. A buyer who needs a week to read a document and consult a partner is a buyer making a decision they will still hold in six months. A buyer who agreed on a call is a buyer who may reopen it on Monday.