The rationale behind tax breaks is to encourage money to be invested into something that benefits the nation. For example, you want cleaner energy, so give tax breaks to those that invest in clean energy technology. Or you give tax breaks for sending yourself or your kids through college, since a college education should increase the quality of the nation's workforce.
In practice, Congressmen sell tax breaks for votes. They vote in tax breaks that will help the industries in their local area, which raises employment, makes the area more prosperous, etc. So if you're a Congressman from a district that has coal mines, you try to convince the rest of the nation's Congressmen to give tax breaks to people that invest in coal powered plants.
So, regardless of the intended rationale, the tax breaks can often go off on tangents and serve no purpose other than to increase the chances of re-election of a few key Congressmen with enough seniority and power to wind up on key committees.
Or, tax breaks can go to people that can offer the most help in re-electing the Congressman, which would be people willing to donate large amounts of money to election campaigns, or at least to political action committees since there's limits to how much money a person can donate to a single candidate.
Or, as WhooWee mentioned, rich people are taxed so much that Joe Six Pack feels sorry for their misery and will only vote for Congressmen that will make a life a little more bearable for the nation's suffering rich. And when people earning 87% of the nation's income have to pay 97% of the nation's taxes, then they're definitely suffering.
That's at least a partly facetious answer. While I couldn't care less about people earning several hundred thousand dollars a year, you don't actually have to earn all that much before you're paying enough taxes per year that you could buy a new car with that amount of money. But I, like every other American, am not rich. Rich people make more than I do, lazy people make less than I do, while I'm just one of those poor, but honest folk that work hard for a living only to see the government take huge chunks of my hard earned money.