@Animandel - The reasons Congress used the impeachment proceedings against Bill Clinton can be found in the last paragraph of this article. That paragraph says that even though members of Congress know they have little chance of actually getting the person thrown out of office, they do have a chance to show that they don't approve of his actions.
As you said, Clinton and the intern were both of legal age, but he still took advantage of a young women who looked up to him, and he was able to do that because of the position he held. That's not the way the office should be use.
The final paragraph goes on to say that impeachment proceedings can alert the public to what an elected official is doing. I think that is good because then the voters are given the chance to decide if they want the person to represent them in the future.