Normally, in a democracy, you have two chambers for the legislature so that one of them is filled by popular vote from all over the country and the other by representatives allocated for administrative divisions.

In the US both chambers are allocated for predefined divisions, just on different scales (state vs slice of population), so the principle of the popular vote is not represented.

It does serve (in theory) to make up for a state that had lower population, but since the slices are subject to manipulation it’s debatable.