Accumulating merit and wisdom by developing universal generosity and creating auspicious circumstances, through the mandala offering
When we take our first steps on the Path, we are not yet capable of helping others. To accomplish the good of others, we must first perfect ourselves, by purifying and transforming our minds. This is the aim of what we call the preliminary practices, which establish the foundations of all spiritual progress. You may feel like dispensing with these foundations in order to practice teachings that you think are more profound, but if you do so, you are building a palace on the surface of a frozen lake.
Dilgo Khyentse Rinpoche
As for the benefits of mandala offering, it is said that it will help to perfect entirely the six transcendent perfections, which are the foundation or support for all the qualities of the paths and bhumis.
The Mandala Sutra says:
Offering the mandala is generosity.
Cleaning it thoroughly is discipline.
Removing any tiny forms of life is patience.
Practising with enthusiasm is diligence.
Meditation is focusing the mind in that moment.
Wisdom is properly drawing the mandala design.