What problem does the Congress solve in the democratic process which happens elsewhere where there is no such thing?

Congress as a whole? I don't know if there's anything unique it solves. It's merely the US's compromise to balance between a monarchy and a weak federal government with little control over the coalition of states.

The big issue is that our House of Representatives stopped being proportional to the population some 90 years ago. I believe analysts suggested that a House today would have over 1000 members, as to the 435 seats today. So that only increases representation of smaller states.