Tree forts are like a safe haven in the woods (i secretly am kinda scared of the woods...well, of bears and cougars that is). I remember pretending I was like the Swiss Family Robinsons, stranded in the wild. How nice would it be to have one of those "forts" in your backyard, camp out whenever you want without needing to set up a tent or sleep on the floor? Your own little oasis in the forest.
...and yeah, that last photo is kinda nuts.