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 New Moon begins the Moon’s Waxing phase, when She grows stronger and brighter, beginning as a slender crescent and building up towards Full. This is the phase of the Maiden Goddess. This is the time for new beginnings, and growth. At New Moon we work for those things we wish to start, and those things which we wish to develop – either over the course of the month, or over a longer term. It is a time to work on qualities which we wish to cultivate, skills we wish to learn, behaviors we wish to perfect, and to focus on our goals.
In ritual, we attune ourselves to these energies, and work with them toward our personal goals.