They're a versitile bike for someone who lives in an area with cold winters and likes to ride offroad a bit. Owning a fat bike has opened up an entire season to more cycling. I used to dread snow as it (mostly) prevented riding. Now, a few inches is something to look forward to. Also, most fat bikes are surprisingly capable and fast trail machines. This hit home to me with watching a tight trail race around here in the summer: the fatbikes were extremely competitive with regular mountain bikes. The guys on fatties were absolutely bombing sketchy descents and not losing much time on climbs.