We have them because the Founders thought it was a good idea.

Then they thought it was a good idea to have two houses with different criteria, giving two chances to quash legislation. Plus the veto. Plus the Supreme Court (whose job they left almost completely undefined, so they made up a job description).

So we have them because we can't get rid of them. But they are practically useless. They can usually manage the bare minimum of reauthorizing the thing they said last year. Then they can rename a few post offices, have some grandstanding hearings, and then go home to campaign for reelection.