"Aadhaar is a voluntary ID scheme. No services are linked to Aadhaar per se."

Though the government stated in court and in parliament, that Aadhaar is voluntary, or it is "just a identity number" not linked to anything, it has over the years, repeatedly made Aadhaar mandatory for an ever-widening range of facilities and services: to receive scholarships, food grain and cooking gas subsidies, access pensions and provident fund, to open bank accounts, to vote, and even to register a marriage, or adopt a child.

It was made clear, life without Aadhaar would be very difficult. In these circumstances, saying that Aadhaar is voluntary is like saying that breathing or eating is voluntary.
This has suited the UIDAI very well, since it has led people to rush to Aadhaar enrolment centres.