I sometimes go for a walk by myself, for no other reason than itself
I sometimes go for a walk with friends, for no other reason than itself
"For no other reason", in this case, meaning not because you're going somewhere, not because you need to walk the dog, not because you want to get fit, and not to achieve something, but purely because you fancied going for a walk and enjoying the whole "outdoors" thing.
(Not that you have you pay attention to the outdoors. Being completely lost in thought while walking is a valid life choice.)