According to Wikipedia, the first number of the Thai ID card categorises the holder into one of the following categories (these numbers correspond to the first digit of the identification number):
0 Foreign worker
1. Thai nationals who was born after 1 January 1984 and had their birth notified within given deadline (15 days).
2. Thai nationals who was born after 1 January 1984 but failed to had their birth notified in time.
3. Thai nationals who was born before 1 January 1984 and had not yet moved their residence.
4. Thai nationals who was born and moved their residence before 1 January 1984.
5. Dual Nationality which born outside Kingdom, or People who missed the official census.
6. Minority or Foreign nationals who are living in Thailand temporarily and illegal migrants.
7. Children of people of category 6
8... 1.. Foreign nationals who are living in Thailand permanently.
..... 2. Thai nationals by naturalization (Sub-sequence from above).
Why was this categorisation introduced? What is the categorisation used for?