I sometimes ride trails that are rough enough that I decided to get a mountain bike. Most of my riding is off the road, but it's gravel or limestone, etc (as depicted below). But then I started doing some single track, and I didn't want it to beat up the NEX fork on my Roam too much. So I bought a 27.5" mountain bike for that. My Roam is used for general riding and family rides, and it has the rack, so I can put my tools and panniers on it and such. My mountain bike is used mostly for single track, but my daughter likes to ride it when we go riding together off road, even when it's not "rough". It has 27.5x2.1" Vittoria Mezcal G+ TNT tires on it that are super comfortable and yet still very fast on smooth surfaces. Great tires, those.
IMG_20180930_130647117 by
jnjadcock, on Flickr