To constrain the executive. 2000-10000 years of history demonstrate that roughly all kings all the time are horrid. They range from uninterested, and therefore only minimally oppressive to interested and(theref0re) atrocious. Democracy is an attempt to constrain the power of kings, whose power is known to be dangerous on a level somewhere between forest fires and Cthulhu.
I assert that while democracy has tried to prevent oppressive kings, it has failed, and seated instead oppressive "elected" leaders. Given that...the founders of the US seem to have understsood the problem better than anyone else. First, set up a power-tension between the states and the federal government...with purse-strings at the state level. Then set up a power-tension between the branches of government. And finally, set up a democratic system, so that the losers who don't accomplish what is actually preferred by the electorate can be thrown out. The problem is oppressive government...and republican federalism with democratic elections is the first large-scale attempt to solve the problem. Seems clearly to have failed...but it is never wise to try to dispose of a system without attempting to understand the problem the system was attempting to solve.