The man shaking hands with a businessman with briefcase. In a business situation, you’re expected to offer a firm handshake to your business associate or client. A firm handshake with good eye contact communicates self-confidence. Handshaking is a form of nonverbal communication that says a lot about a person. For example, an overpowering handshake can indicate dominance or control. A weak handshake can indicate insecurity, disinterest, shyness, and aloofness. An awkward handshake indicates nervousness or a lack of social skills, which in turn reflects on credibility. Although, handshake is a customary greeting, it makes the first impression on a person, which is extremely important if you are attending a job interview or a business meeting. Apart from giving a firm and strong handshake, it is also important to position your body correctly and make eye contact.
In short, if I'm not wrong, they're doing a public sign of completing a business or diplomatic agreement.