The primary reason: to be able to phone and be phoned while not at home. If you don't need that, don't buy it, unless you want to use it for (for example) listening to MP3, sending and receiving SMS (text messages) or taking pictures.
I've got one myself, and use it about twice a month, to tell my wife I missed the train and will be home later. And about once a year my work wants to be able to reach me during the weekend, and I prefer not to stay home the whole weekend, so I switch it on and take it with me then.
Is there any good reasons for me to get a cell phone? Everyone has one, but any specific reasons?