It would be amazing if someone could figure out a way to measure constituent opinion that didn't involve people showing up at or calling in to an office.
Well, that's kind of the thing. Everyone's willing to kill and die for their beliefs on the internet; a lot fewer will actually campaign and canvas and march and vote for their beliefs. The inconvenience of calling or mailing your politician tells them that you have passed a rock bottom threshold of willingness to act, and might conceivably have an effect on their vote totals next election.