Why 2 Houses of Congress?

Why have two houses?
The House of Representatives allows for simple majority votes.  It allows issues to be brought forth and passed based on a simple majority representation of the population.  That can be good, but it can also let one state, such as California, have an overwhelming voice that drowns out smaller states - such as Rhode Island.

The Senate ensures that each state has equal representation - and an equal voice - in our federal government.  With unlimited debate, the senate can slow down government - and make it harder for changes to occur.  It also makes sure that any new laws are perceived to be good for the absolute majority of states prior to passage.