Liberty is best for civilizations which desire rapid expansion.
Adopting Liberty will provide 1 Culture in every city. Unlocks building the Pyramids.
Adopting all policies in the Liberty tree will grant a free Great Person of your choice near the Capital.
As a policy, liberty can be defined as the basic assumption that a nation's citizens deserve to be free. Free from physical restraint, and free from despotic control. Serfs, slaves, and hereditary servants do not enjoy liberty, nor do citizens who are forced to live within a certain village or gulag or ghetto.