Let's say you like the look of a house that is for sale. You judge it is worth - for argument’s sake - 100 rupees. The chances are the seller will tell you he will only take, say, 50 rupees as a formal payment and demand the rest in cash… It means the seller can avoid a hefty capital gains tax bill. Buyers benefit too because the lower the declared value of the property, the lower the property tax they will be obliged to pay.
This means that Indians tend to have much smaller mortgages compared to the real value of their properties than elsewhere in the world and hence the system is more resilient against shocks to the system. Of course, the system concentrates wealth with rich people who can afford to pay cash, but the point made in the article holds.