A tree with a smooth trunk falls due to some natural process of weather or erosion, and comes to rest in a nicely horizontal orientation with its top surface a convenient two feet or so above the ground. As long as it persists in this state unobserved, it remains a fallen tree trunk and cannot be anything else.
When observed by a human being, it continues to be a tree trunk, but in casting the physical reality of the trunk into a metaphysical representation of a trunk in their mind, the human may think to themselves "that fallen tree would be a nice place to sit". Thus, human observation gives the trunk the potential to become a seat. It is only a seat in fact when someone is sitting on it, and it reverts to being a trunk with the potential for seat-hood once the sitter rises to their feet.
A constructed bench, on the other hand, is always a seat, even when un-sat-upon and unobserved. Unlike the trunk, which becomes a seat temporarily because of the metaphysical manner in which humans interpret the physical world, the constructed bench is a product of human beings intervening in the physical world, and all such interventions consist of the imposition of metaphysical concepts upon the physical.