IPv6 was initially designed with a compelling reason in mind: The need for more addresses.
This need was due to the vision of the Internet grow, where billions of new devices (e.g., cell phones, PDAs, appliances, cars, etc.), billions of new users (e.g., in China, India, etc.) and new ³always-on² access technologies (e.g., xDSL, cable, ethernet-to-the-home, etc.), were already becoming a reality.
Of course, there may be alternative technical solutions, such as NAT (Network Address Translation), but they won't work so easily to allow this grow, new enhanced applications and services, and in general the innovation. Furthermore, those techniques make Internet, the applications and even the devices more complex and this means a cost increase, while IPv6 can make, in the medium/long-term, every IP device cheaper, more powerful, and even consume less power (which is not only important for ecologic conservation, but also to have longer battery in portable devices, such as cell phones).
Consequently the design of IPv6 was an opportunistic way to improve Internet, with new benefits such as:
Expanded addressing capabilities.
Server-less autoconfiguration ("plug-n-play") and reconfiguration.
More efficient and robust mobility mechanisms.
End-to-end security, with built-in, strong IP-layer encryption and authentication.
Streamlined header format and flow identification.
Enhanced support for multicast and QoS.
Extensibility: Improved support for options / extensions.