Guarantees the states a republican form of government - congress allows senators and representatives from a state to take their seats in congress. This is in recognition of the States' republican form of government.
Guarantees protection - any attack on a state is deemed an attack against the United States of America. Thus, each state is under the protection of US Armed Forces, Navy Seals, etc.
Guarantees territorial integrity - the national government can't create a new state in the territory of an existing states. Borders are recognized to differentiate one state from another.