I keep getting letters and postcards urging me to vote. It's not specifying why or for whom, just says "vote!"

I keep asking myself - why are they (who?) interested in people going to vote? Why are they spending millions of dollars on these campaigns, what are they standing to gain?

I'm going to vote, so that's not a question. Just trying to understand these groups who want people to vote. 

Even if some people don't vote, wouldn't the interests of the nation be properly (statistically) represented by the people who do get out and vote?