Not-so-long ago there was no such thing as suspension on a bike. Yes, there were springs but they could hardly be considered suspension.
Hardtails are lighter and allow more of the riders energy to be transferred to the ground. I would say that 99% of riders really don't need full suspension, but somehow they believe that it helps and that's cool if thats what you want.
Suspension is to keep the wheels on the ground for better handling and power transfer, not necessarily for comfort.
Think about it this way... a luxury car has very plush suspension to keep the occupants comfortable. They are also big and heavy and eat a lot of fuel. Jeeps are rough and tumble and tend to be harsh riding on the road. They have very stiff suspension but are at home playing in the dirt and they usually have pretty good fuel economy. Part of the reason for this disparity is because of the suspension.
Now, it also wasn't too long ago that I, a bike designer, was of the opinion that suspension was for people that lacked proper technique- we have suspension built into our bodies (elbows, knees, etc) and just have to learn to use it. I have since developed a keen sense of propriety and utility and have changed my opinion that suspension, especially a good fork, greatly enchances the riders abilities. Full suspension is good to, for those that need it and understand its application.