Port application / windows firewall

Could you please help me?

I am using windows firewall advanced settings and I don't understand what is the meaning of blocking '' only one port of the apllication instead of the total access of the application to internet''

Does it have any sense at all?

Let's assume I have skype as an application, right? In which cases (for security purpose) in your day job as admin you choose to allow the traffic only tru one port, blocking instead the traffic to another port?