The field makes energies of the Shatachandi ritual - available to us.
It is not the same as directly invoking the Goddess. The field is a safer way to approach such a potent ritual - avail the benefits without the associated risks.
Think of Angelic Intercession. Is it same as invoking an angelic being to appear before you through a complex ritual like the Abremalin Operation which only the most advanced of practitioners should attempt? Not really. It is a gentle way to seek help form Angelic beings - you may feel their presence, and most experience their grace.
Chandi is Kundalini personified in a blazing state piercing through the heart knot. This is why caution is required when invoking forms like Kali, Chandi etc., to ensure sufficient prep has been done. Hence we say - Tantra is like a jet. It flies very fast, but without the needed skills, an accident can be fatal.