First, it helps put actual weight behind their words, so they can say "I received 2000 calls about this issue in the past week" or something. Second, members of congress have limited time and limited political capital, so if for example you want them to prioritize stopping the Muslim ban over stopping the border wall, you should let them know. It helps them choose what to focus on, and can also help them recruit other members to help them if they can say "look at the numbers, people really care about this one".