Pair protocol is the simplest and least scalable scalability protocol. It allows scaling by breaking the application in exactly two pieces. For example, if a monolithic application handles both accounting and agenda of HR department, it can be split into two applications (accounting vs. HR) that are run on two separate servers. These applications can then communicate via PAIR sockets.
The downside of this protocol is that its scaling properties are very limited. Splitting the application into two pieces allows to scale the two servers. To add the third server to the cluster, application has to be split once more, say by separating HR functionality into hiring module and salary computation module. Whenever possible, try to use one of the more scalable protocols instead.