The purpose of the porch and its Caryatids is unclear, but it has been suggested that it served as a bridge between the newly constructed Erechtheion and the Old Temple of Athena, thus building a bridge between the old and new home of Athena. Some have also suggested that the Caryatids served as a marker to the tomb of Cecrops, believed to have been located below the porch.

The Caryatids is not a unique feature of the Erechtheion, and its origins are unclear. The most famous explanation of the Caryatids origin is that of the Roman Architect Vitruvius. He writes:

“Should any one wish for information on the origin of those draped matronal figures [...] called caryatids, I will explain it by the following story. Carya, a city of Peloponnese, joined the Persians in their war against the Greeks. These in return for the treachery, after having freed themselves by a most glorious victory from the intended Persian yoke, unanimously resolved to launch a war against the Caryans. Carya was taken and destroyed, its male population extinguished, and its matrons carried into slavery. To ensure that these circumstances might be better remembered, and the nature of the triumph perpetuated, the victors represented the matrons draped, and apparently suffering under the burden with which they were loaded, to expiate the crime of their native city. Thus, in their edifices, did the ancient architects, by the use of these statues, hand down to posterity a memorial of the crime of the Caryans.” (Vitruvius, On architecture 1.1.5).

The current models are replicas. Five of the originals are displayed at the New Acropolis Museum, and one at the British Museum in London.

Erechtheion.org
Ancient-greece.org
Wikipedia: Caryatid
Michael Lahanas: Greeek architecture: The Erechtheion