The photo shown is of a roof connection made from one building to a shed that was built next to it. The work was done by a handyman who doesn't know much about roofing.
This connection is sure to fail and the result will be water intrusion at the joined area. The corret way to have done this connection would have been to add metal flashing that went over and under the connection. The black roof mastic that he buttered on the joint will harden and crack. Had he used metal flashing the water would have only one way to go and roof mastic would not be a factor. I've met and in fact I know some handymen who are very good but most of the time, even if you get a good one who does neat work, the work will be done incorrectly.