How do you solve a problem best? In your solitude alone or in a group? I know that I solve problems better in a group. The group has to be small so you don’t get disturbed. But it has to be large enough so a discussion can occur.
Programing is not about writing text fast. Typing is never the bottleneck when programming. The bottleneck is problem solving and solving the right problem.
Pair programming is about two persons that solve a problem together. Problem solving in a group tend to result in better solutions. Different people see a problem from different angles. The result is usually better than a solution that one person came up with.