Why did the Founding Fathers create two houses in the legislative branch?
To properly represent the people at one level and the states at another.
The founders realized that even within the 13 colonies the population was spread disproportionally which meant the more populous states tended to be more powerful.
Article 1 Section 2 provides for these:
The house of representatives was meant to give a particular number of residents in any one state just as much say as the residents in another. The original number was one representative for every 35 thousand residents. That, of course, has since been modified, greatly.
As a balance the give each state an equal footing in at least one forum, each state must have 2 senators.