From Hiking Trip Reports:
… Duct tape is probably the most versatile item you can carry when hiking. It can mend and repair almost anything from blisters to holes in air mattresses. …
Hiking Trip Reports – Duct Tape: The Hikers Friend
related – 10 hiking essentials
besthike.com 
The Trailspotter’s Guide To The Galaxy concurs, saying that a good trailspotter should “always know where your duct tape is”. It goes on to suggest that “instead of carrying a roll of duct tape, wrap lengths of tape around your water bottles and save valuable backpack space”.