Politicians need lots and lots of money for their election campaigns. Corporations want legislation that discourages competitors and boosts profits. Lobbyists, with the help of campaign finance laws, provide the match making between the politicians and corporations.
The executives of a corporation hire a lobbyist. The lobbyist figures out which Senators and Representatives can write and/or get legislation passed. The executives (and underlings) donate money to those Senators and Representatives and even the President in order to further that process. Because these donations are public information, the lobbyist is able to compile a list of donations from the employees of that corporation and calculate a total. The lobbyist goes to the politicians and points out the "generosity" of the corporation. The politicians push legislation through that benefits the corporation.
The politicians, the lobbyists, and the corporations win. The taxpayers lose.
There are some interesting subtleties. The corporation does not demand or require that anybody donate to various campaigns. That would be illegal. Indeed, they may not even suggest it directly. Instead, it's known through the grapevine (but certainly not officially) that your campaign contributions will be monitored (and can be because it's all public information).
When describing corporation strategy and business development in meetings and in company newsletters, the politicians that can help the company are identified. You then donate to those campaigns. You get raises and bonuses that are partly based on reimbursing you for those political donations. Again, not officially, but that's how it works. You don't have to do it, but it will adversely affect your compensation and career if you don't.
It's funny to me that most people think that making donation information public makes politics somehow more fair when, in fact, the whole purpose of doing it this way was to enable corporations to direct their executives and employees to support various candidates. If campaign contributions were secret, it would be much harder to do.
That's why a Media Matters "correction" (via an Instapundit post and Reason article) is so silly. The "correction" was that various articles in the main stream and non-main stream media were incorrect when they stated that Obama received a huge amount of money from BP. According to Media Matters, the money didn't come from BP, but rather from employees of BP.
It's pretty much the exact same thing!