Where I live we have very long winters and it seems like spring will never get here. If you wait until the last frost in the spring, and plant anything from seed, you just don't get a very long growing season.
Since we weren't going to be moving anyplace that was warm, I figured the best way to deal with this was to build a greenhouse. We looked at several different green house plans, and decided to go with a simple structure that was easy to put up and not too big that I would get carried away.
Being able to start my vegetables and flowers earlier has made such a difference for me. As long as I live in this climate, I will continue to use greenhouse gardening to get a jump start on my growing season.