benefits of personas

uie (jared spool) article Three Important Benefits of Personas

1. Helps to separate you from the work - is it natural "ground" yourself into the work (how you would use the interface) and personas helps designers be more objective and break away from that natural behavior.
2. Helps provide a shared vision through storytelling - putting the personas into a story (scenarios) helps designers communicate the details of the project to the team.
3. Helps solidify the shared vision through role playing - (I see this benefit as an extension of #2 and not separate)

but you have to create good personas to get the full benefits

To get the benefits, the personas have to have rich, relevant detail. They need to accurately represent the users the team is aiming for. And they need to have a solid foundation in the experiences of real users to be believable and meaningful.

in addition, the personas should be continually reviewed and modified throughout the project as your knowledge of the users grows. for example, results from usability studies should provide insights to help refine the personas. Personas should be treated as

living descriptions — constantly changing as they learn new things from their ongoing research, studies, and design exercises.