Why IPv6? (Theoretical Reasons)

only compelling reason: more IP addresses!
for billions of new users (Japan, China, India,…)
for billions of new devices (mobile phones, cars, appliances,…)
for always-on access (cable, xDSL, ethernet-to-the-home,…)
for applications that are difficult, expensive, or impossible to operate through NATs (IP telephony, peer-to-peer gaming, home servers,…)
to phase out NATs to improve the robustness, security, performance, and manageability of the Internet

only compelling reason: more IP addresses!