Simple: so they'll know.
Not everyone is fortunate enough to live in a state where the Congresscritters have been educated (read, "beat over the head") to know that the Second Amendment is sacred.
Maine, for example, is represented by at least one senator who is most charitably called a RINO, and would otherwise vote with her liberal Dimocrat pals on an issue like this.
If you don't tell them, how do they know?
And trust me, a phone call is worth a hundred emails...
They know that people who take the time to contact them are very likely to vote.
We just defeated an anti-gun bill here in New Mexico (HB 44) by testifying at hearings, sending emails, calling, faxing... Several members of the committee in which the bill was killed told me that all the communications gave them the political backing to confidently stop this "gun control legislation."
The reason so many elected reps seem unresponsive is a self-fulfilling cycle. The less people engage, the more these reps feel they can act with impunity. The more communications they receive, the more responsive they become to their constituents.
Saying it won't do any good is just an excuse not to take a few minutes and put pressure on elected reps. As you sow so shall you reap. Quit complaining and do something.
And yes, phone calls and written letters are the most effective forms of contacting elected reps.