Personas are a great way to help you focus on real users during design.  They are concrete descriptions of the users and what they need to do.  Besides enabling you to keep the users in focus, they can help you identify where not to focus.  They are a tool to help you prioritize functionality and think through how the user needs to accomplish their tasks.

Common sense tells us that we need to create products that can accommodate the needs of as many people as possible.  That means more people will be able to and want to use our product, right?  No!  In reality, designing a product that works for everyone, means it likely will not work for anyone.  Since everything on a webpage or in a web application competes for users' attention, all the pieces that are not relevant to a particular user increase their cognitive load, and information overload and overhead.  By designing for everyone, everyone ends up paying this high price. 

The key is to identify the group of users whose goals and needs we have to meet and will also still meet the majority of other users goals and needs.  This group will be represented by the primary persona.  Once you choose this primary group of users, you can focus design on them while keeping in mind the secondary users.  See Type of Personas for more on primary and secondary personas.

Strengths of Personas in Design

According to Alan Cooper, et al in "About Face 2.0" (p. 55), the strengths of personas as a design tool are to:

• Determine what a product should do and how it should behave. Persona goals and tasks provide the basis for the design effort.
• Communicate with stakeholders, developers, and other designers. Personas provide a common language for discussing design decisions, and also help keep the design centered on users at every step in the process.
• Build consensus and commitment to the design. With a common language comes a common understanding. Persona reduce the need for elaborate diagrammatic models because, as the authors have found, it is easier to understand the many nuances of user behavior through the narrative structures that personas employ.
• Measure the design's effectiveness. Design choices can be tested on a persona in the same way that they can be shown to a real user during the formative process. Although this doesn't replace the need to test on real users, it provides a powerful reality check tool for designers trying to solve problems. This allows design iteration to occur rapidly and inexpensively at the whiteboard, and it results in a far stronger design baseline when the time comes to test with real users.
• Contribute to other product-related efforts such as marketing and sales plans. The authors have seen their clients repurpose personas across their organization, informing marketing campaigns, organizational structure, and other strategic planning activities. Business units outside of product development desire sophisticated knowledge of a products users and typically view personas with great interest.

Personas can also help ensure the entire project team is on the same page about who is the focus of design.  The term "user" is ambiguous and each person on the team probably has a slightly different picture in their mind of who the "user" is and what they need from the product.  Personas create a concrete shared understanding.

They also keep us from designing for ourselves.  Cooper calls this self-referential design.  It is natural to think about what we need or how we would perceive certain aspects of the product during design.  However,  neither the designers nor the rest of the product team are usually the typical user of the product.  We can use personas to "get out of our own heads" so to speak.

For more on the strengths of using personas in design, check out "About Face 3.0", Cooper, et al (or "About Face 2.0").

Using Personas in Design

Use Personas to plan your product

• Brainstorm possible features and functionality using your personas. 
• Prioritize functionality based on your personas' needs.  A weighted priority matrix can be used to identify the importance of functionality. For an example, see the Fluid Use Case Matrix created for a research project on Content Management in Higher Education.
• Analyze similar products through the eyes of your personas to get ideas about what you do and don't want to do in your design.

Explore design solutions from your personas perspective

• Identify the use cases your design must support
• Create scenarios for your personas to understand how they need to get their work done and in what context
• Explore Mood boards and visual design with your personas in mind

Evaluate your solutions from your personas perspective

• Complete cognitive walkthroughs and design reviews from your personas perspective
• Use personas to help you create user testing scenarios and think about recruiting participants
• Focus QA testing and create persona-based test cases, persona bug labeling (23 Joe bugs, 43 Susan bugs))