Mobbing is when a whole team sit around one computer similar to pair programming and all code together! This practice like pair programming has many advantages and even though it seems like it would slow you down having everyone at the same computer it can actually speed you up as there is a lot more knowledge sharing and focus on the task at hand.
Mobbing is not five people watching and one person coding, like pairing the people who aren't at the keyboard (driving) should be navigating (making the decisions and reviewing the work being done).
Mobbing can be useful for complex tasks and knowledge sharing, making sure that all the people who are required to solve a problem are sitting around the computer. This should make for no wait time when things people have questions.
Mobbing can also be used for learning, grouping together to pick up a new technology or practice. Think about it one the whole team will have a shared understanding rather than one person being a single source of knowledge and failure.
Best of all mobbing makes sure that the work being done is the best the team can do, with every members skills being used and making sure that and problems members have should be spotted by the other members.