Site-Local/Unique-Local Addressing (IPv6)

[Nicholas Warren <nwarren () barryelectric com>]

Layman here, I was reviewing RFCs for a local address for IPv6. I came across two RFCs that seem interesting.

3879 Which deprecates Site Local Addresses.
4193 Which seems to add Unique Local Addresses.

What is the main difference here? Why was this standard removed then added back?

Thanks,
Nich Warren