The senate is supposed to represent the state governments at the federal level, but they stopped doing that with direct elections. It was to prevent large states with a concentration of population (currently like CA, NY, TX, FL) to have too much power over smaller states.

Here's some background on the arguments for and against. It's got some partisan hyperbole so try to ignore all that.

https://www.theusconstitution.org/news/the-17th-amendment-un...