As the solution to city-state warfare, the Hoplite was a heavily armored warrior-citizen who went to war when the city-state called upon him.
These warrior-citizens had to purchase and maintain their own armour and weapons since the state could not provide them.
Many of the people that were called to war could not afford heavy armour, and wore only a shield, a spear and a leather helmet. The leather helmet was more comfortable than one made out of metal, and affordable. Greaves were used rarely, as they were heavy, uncomfortable and reduced mobility. We commonly refer the to the shield as "hoplon" nowadays, but the Ancient Greeks called it "aspis". [Ta hopla] meant "weapons", including armor.
Only till the later years of Ancient Greece did these warriors replace their heavy armour for lighter armour allowing better mobility against skirmishers and missile-armed troops like peltasts.