Evaluators like graphics that let help them see the points you are trying to make instead of making them work too hard to get the point on their own. But maybe the biggest reason for using graphics in your proposal is that at least one of your competitors will use graphics in their proposal. It depends on the message you want to send. Take a look at this list of typical proposal messages and the corresponding graphics that can best convey them. Readers remember concrete images better than they do abstract concepts.
Graphics lure the reader to the page.
Graphics bring the abstract and the concrete together. Readers expect to see graphics in proposals. Graphics require less energy to understand than abstract ideas. Graphics help you structure and present thoughts, ideas and concepts. Just about any sort of graphic you would put in other technical documents would work in a proposal.