What is the purpose for the length of a Senator's term of office?
This motivates the Senator to be honest and to work for the people or state that elected them. They won't get too comfortable in their position.
I think there are two main reasons for having fixed terms for Senators. The first is to ensure that the people get to decide who represents them from time to time. If a Senator were there for life, you wouldn't be able to get rid of a bad one. The second reason is because the chances of becoming corrupt are greater if you can't be removed from office.