There are probably as many reasons to do an Ironman as people doing it, but for me these are the most important

Photo by: Bakke-Svensson/Ironman.

Test your limits: With my first Ironman I really wanted to see if I could do it. And my first Ironman was on the limit
Allround training While Triathlon builds on the swim, run and bike the whole body is involved in doing it. Lower body in cycling and running, upper body in swimming.
Variation Instead of doing only one sport, we do three. It makes it easier to motivate one self and makes it possible to train a lot more as for only one sport.
Community The atmosphere on race day is tremendous. This is one of the greatest reasons I’m still doing it.
Independence This is a sport where you don’t have to be a member in any sports club to do it. You can do it alone if you want to. And you can do it directly from home. Just go out and run or bike. Only for the swimming, it’s different.

Here is a link to another bloggers 5 reasons to do an Ironman.