There is an interesting story in the MATSYA PURANA in which the birth of the Vastu Purusha is narrated. By reading that story one knows why the worship of the Vastu Purusha is necessary before beginning constructing any house.

While fighting with the demon, Shiva was very much tired and began to sweat profusely. A man was born of the drops of Shiva’s sweat.He looked very cruel. He was very hungry. So he began to make penance to appease Lord Shiva and get a boon from Him.

Shiva was pleased with his penance and appeared before him. The devotee prayed to Shiva, “Oh Lord! Please permit me to eat away all the three worlds.” Shiva said, “Let it be so.” The devotee’s joy knew no bounds. He got possession on all the three worlds and first he was ready to eat the terrestrial world. Then the celestial beings, Brahma, Shiva and the demons (Rakshasas) also were terrified and caught hold of the devotee encircling him.

Being bound like that, the devotee lay down there only. Since then, he has been lying there surrounded by the celestial Beings and thus he was called “the God of Vastu or VASTU PURUSHA”.