Economics:
Pairs spend about 15% more time on programs than individuals. However, the resulting code has about 15% fewer defects
Design Quality:
A system with two programmers possesses greater potential for the generation of more diverse solutions to problems for three reasons: (1) the programmers bring different prior experiences to the task; (2) they may access information relevant to the task in different ways; (3) they stand in different relationships to the problem by virtue of their functional roles.
Satisfaction:
the pairs enjoy their work more because they are more confident in it
Learning:
shared between pair programmers, from tips on programming language rules to overall design skill.
Team Building and communication: allows team members to share problems and solutions quickly making them less likely to have hidden agendas from each other.