Hadley explains (at the above link):
The dplyr package makes each of these [data processing] steps as fast and easy as possible by:
- Elucidating the most common data manipulation operations, so that your options are helpfully constrained when thinking about how to tackle a problem.
- Providing simple functions that correspond to the most common data manipulation verbs, so that you can easily translate your thoughts into code.
- Using efficient data storage backends, so that you spend as little time waiting for the computer as possible.
The dplyr debut blog post may also be of interest.