You must have seen that when a person starts building a house on a vacant earth, the land is worshiped before laying the foundation.
According to Vastu science and scriptures that there is a Patal-Lok under the land, whose lord is Sheshnag. Sheshnag is a servant of Lord Vishnu. He is the one who has lifted the earth on his hood. At the time of Bhoomi Poojan, the purpose of worshiping the silver snake in the foundation is to find the grace of Sheshnag.
By placing a silver snake in the foundation, it is believed that the rest of the snake should take care of his house as well as the rest of the earth. The building will be safe and longevity.
The same faith and belief works behind placing the Kalash (urn) in the Bhoomi Poojan, that by this, Sheshnag will get the grace of God. According to the scriptures, Sheshnag resides in the Ksheera Sea, so by adding milk, curd, ghee to the kalash, the Sheshnag is invoked by the mantras in the kalash so that the Sheshnag gets the direct blessings of God.
By putting coin and betel nut in the Kalash, it is believed that Lakshmi and Ganesha will be blessed. The Kalash is considered to be the symbol of the universe and the form of Vishnu and he is prayed to remain seated in this land along with Goddess Lakshmi and Sheshnag always support the house built on land.