The Union is predicated on the States agreeing to certain terms and conditions that guarantee certain State Rights while compromising on others.
One such compromise is representation within the Union, whereby the Lower House (House of Representatives) has the States represented proportionally by population and the Upper House (Senate) has the States represented equally regardless their population. Territories that aren't a State receive no representation.
Additionally, the Lower and Upper Houses each have different duties and powers afforded to them. The Lower House legislates matters concerning money, among other things, while the Upper House legislates matters concerning government appointments and foreign diplomacy (eg: treaties), among other things. Both Houses must also agree with each other on any bills that are intended to go to the President for signing into law.
The States in the Union are tantamount to independent countries in most other contexts, so States' Rights are a very big deal.