Let me approach this from a couple of angles.
First, for myself...I go to the outdoors, backpacking specifically, to unwind, rejuvenate, unplug, commune with nature and recharge. Like a lot of people, I work in an office and sit in front of a computer all day. Getting out allows me to "escape". I purposefully leave my mp3 players, cd players and radios at home. I love music, but when I hike I want to leave that all behind.
I tend to "zone" out while hiking. My brain disengages and wanders where it will. Sometimes that will be working out problems at home/work, but usually it just runs wild.
Second, I am a Scout Master for a local Boy Scout Troop and I often take a group of boys out for backpacking. Their responses constantly amaze me. We do not allow electronic devices, except for GPSr's. At first, the enforced disconnect from the technological world usually had them complaining. Now, the older boys insist on it.
I took a group to Philmont, NM in 2005, mainly 15 year olds. I overheard them on the return talking to others about the "favorite" part of the trip. I expected some of the activities to be high on the list, burro racing, shooting, etc. But no, almost everyone talked about the sunrise from the ridge, or the number of stars. They talked in hushed, awe filled voices. Sure they overcame hardships. WE (adults) were led by them. They had to make the decisions. We did not rescue them. They misread maps, got "lost" for a while, but overcame every obstacle.
Therapy for the boys was growing in self-reliance. Therapy for me was getting back in touch with the quietude of nature. We shared and grew together.
Dang, I can never get it right. Every time I try to explain to family or friends, I only get part of why I do it. I do know it is therapeutic for me. I do know that if I don't get out for a period of time, I crave it.
Maybe it is an addiction instead of therapy.