Personas are useful on large scale projects where direct access to end users is limited or not possible and where.  Additionally, they are helpful for interface designers as well as visual designers when composing aesthetic components to keep in mind who they are designing for. 

Personas capture a profile of a typical end user as well as his/her context of use with the system.  They capture the over arching business context for the user as well as the typical interaction scenarion and constraints.