I used to carry a very small monocular in a little zipper case in my vest for a long time for looking at insects and other stuff. I can't say I used them much, but they were cool.
When I abandoned the vest I also abandoned the monocular and a bunch of other junk I carried for insect collection and identification because I adopted a much simpler system...
"Small yellow bug, small brown bug, larger tan bug, etc..."
These days all I carry is a nice pair of binoculars in the back of my SUV. I use them for occasional scouting of the landscape and looking at women swimming at the various PA State Parks I frequent.