The purpose of the Articles of Confederation was to give the US citizens the freedom they needed after obtaining their independence from Britain. The Articles of Confederation were rejected because they were failing. The weaknesses of the Articles of Confederation were:
Most power was held by states
One branch of government
Legislative branch had few powers
No executive branch
No judicial system
No system of checks and balances
In order to run the country more properly, they founded two more branches (executive and judicial), they added the system of check and balances (that divided the power between the branches in a well-balanced way)