The following is an extract from Lesson 1 of An Introduction to the U.S. Congress. You can get the whole thing here: https://cffad.org/topic/#congress-part-1
Why do we need Congress?
We need a way to solve national problems that can take into account the interests of all the people, with full accountability to all the people, and without majorities oppressing minorities.
Every country has some problems that can’t easily be solved by just a few citizens or businesses, nor by just a few state governments.
- Some problems require a national response, like defense against foreign enemies and border security. United we stand, divided we fall.
- Some problems require laws that are binding on all state governments. For example, free trade requires each state to refrain from obstructing commerce between citizens and firms of different states.
- Some problems require laws that are binding on all citizens. An example would be prohibitions against enslavement by any person or business. Another example would be the need for all citizens to honor patents and trademarks.