If we took that attitude in everything we'd never get anywhere, like just playing 1950s era guitar designs and tube amps. (Oh, ... wait...!) A clear purpose is to see what we can learn that we didn't expect about the solar system, comparing planets, etc. You don't know what you don't know. The technology push is worth something, as well. Communications theory was pushed pretty far with the Voyager program, for example. BTW, NASAs budget is 0.48% of the Federal budget, according to Google. That's half a penny for every federal dollar spent.