To understand the purpose of the Esbats you must understand the Lunar phases.
The Moon represents the Goddess, and also the Soul which has its origins in the Goddess. The Moon represents our emotions and inner self, and the psychic and spiritual energies associated with the Higher Self. Consequently it is believed that our psychic and magical abilities are affected by the phases of the Moon, being strengthened as the Moon Waxes and grows Full, and turning inward when the Moon Wanes.
The Full Moon is when the light of the Moon is at its height in the middle of the cycle. This is the phase of the Mother Goddess. The Full Moon is generally considered to last for three days –one day to either side of the calendar date. This is when the energy of the Moon is at its strongest, so it is a time to take advantage of the extra energy and work on those things we most wish to manifest, since at Full Moon the Lunar energies are pulling with us.
In ritual, we attune ourselves to these energies, and work with them toward our personal goals.