Personas are a very useful tool to help the team keep the same "true north" during a project. Most of the personas I've created are under NDA. The above samples are from a tablet research project and have been scrubbed.
I've come into projects where personas do not exist and because of that who the team was designing for, and what that user needed was unclear. Proposed app and website functionality ends up being based on internal assumptions and not the biggest pain points for our users.
When personas exist user needs in the experience become clear, and their life "outside of the screen" remains in focus. They empower a cross-functional team to create a list of features and enhancements that truly match what the user needs.
On this particular project we needed to show what Becky's day looked like. She was a user that carried her tablet with her everywhere so her location mattered quite a bit. What activities was she doing with her tablet outside of the house that she wasn't doing inside the house? Did she have a data plan so that her tablet was always connected? Were some of the key questions.
We also worked to establish how Becky got around. Did she drive? (rendering a tablet less useful) Was she on a bus? (and therefore able to look at a screen) Insights on all of these items helped us paint a picture of Becky and what types of apps would be useful to her.