the last gesture, the last brushstroke, the last element of the sculpture added, provides the keystone without which the arch of the composition falls.
A piece of music which does not end, may as well have not been played.
A beginning can be be strong, to get you started, or in, but you’ll get by if the climax works.
The working end allows you to step out of the transcendent moment, to appraise the experience. The end allows you you look back at something from which you have now become separated.
The last part is not necessarily a conclusion but does function in a similar way.
Likewise a joke without a punchline is just a story.
Ideally, or often, the last mark, the key mark, is concealed and only the most hardened viewer will spot it.
The last mark, the last gesture, the final element is the move without which the match is declared a stalemate.