This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
Because it's generally good practice to make all instance variable private and access them via methods. This is information hiding - you're separating the implementation of the class from its public interface. This reduces coupling, and makes your code much more flexible. Maybe you'll never need the flexibility in this case, but you can never tell in advance where you will need the flexibility. So it's better to get into the habit of doing it right all the time.