Why is is bad to leave your horse’s boots on?
I have boots for my horse that I strap on his two front legs, and the boots go up the fetlock. But, I forgot to take them off once, and got a “talking to”. Why is it bad though? 
I know I just went off to lunch..and came back fifteen minutes later….and my instructor just told me to always take off the boots after riding.
1) Your horse could get the boots caught on something. At best, you’d ruin a (probably) expensive pair of boots; at worst, your horse could give himself a nasty injury.
2) They could chafe his legs, or possibly even cut off the circulation since, unlike stable bandages or therapeutic boots that ARE designed to be left on, the straps may cause uneven pressure points.
This ONE momentary lapse is unlikely to have any ligering adverse effects, however; I’ve no doubt you’ll remember to remove them next time.
EDIT: Just 15 minutes, and you came straight back? And left him in his stall the whole time? In that case, I think your friend was unnecessarily fussy. I was under the impression you had left them on overnight!
“Dont Stop Believing” [ Amber + Bonita ] Jumping and Dressage Highlights