Any Puja is done to improve the harmony, peace and happiness in our lives; so does an engagement puja.
Many versions of the engagement puja also include the Graha Shanti Havan, it safeguards not only the couple but also both the families.
The engagement puja is a predecessor to the Vivah Puja and is done to bring a deeper and loving understanding between the couple and also their families.
Because it is connected to astrology, this Puja understands the placement and movement of various planets and their effects on the couple and their marriage.
The engagement ceremony is always held before the wedding ceremony. The gap could be that of years, months, days or even hours.
Many communities, in fact, hold the engagement puja on the same day as the Vivah puja, just a few hours before the latter.
The art of performing a Puja or a Ritual at Hindu households has always remained a matter of concern unless you have a "Family Pandit".