The problem with spontaneous agreement is that explicit agreement gives a chance for parties A and B to exchange hidden information without letting third parties know.
The reason for agreements being formalized is also because since you need to negotiate to reach the efficient settlement, since you have to reveal secret information, but publicly implementing the results of secret negotiations leads to suspiscion of secret alliances. If your public allies think you have secret allies, they are not happy, since that is bad for them. So publicizing agreements makes you more predictable, which is good for you.
The problem with spontaneous agreement is that explicit agreement gives a chance for parties A and B to exchange hidden information without letting third parties know.
The reason for agreements being formalized is also because since you need to negotiate to reach the efficient settlement, since you have to reveal secret information, but publicly implementing the results of secret negotiations leads to suspiscion of secret alliances. If your public allies think you have secret allies, they are not happy, since that is bad for them. So publicizing agreements makes you more predictable, which is good for you.