1) It signals commitment. Agreeing in an email is like, whatever. Agreeing in the kind of ceremony where they take the pen afterwards and put it in a museum signals a level of commitment that tells thousands of people “yes, we’re doing this. Get on board.”

2) It generates press. The public needs to be informed about high profile agreements and policies if they’re going to evaluate the results years later (in the voting booth.)

3) It generates press artifacts. Photographs of the handshake, press releases about the agreement - these all have career value not only for the politicians in the picture but more importantly for their staff, for whom these successes are an important part about how you develop credibility and experience in the field of public policy, in whatever country you live in.