Over 2 1/2 lines, Shakespeare builds a metaphor. He creates a miniature portrait of a real-world situation that captures the essence of the experience he's describing.
The experience he's describing is the passage of time, and the feeling of pointlessness and futility that comes with it:
"It's like when a bad stage actor comes on stage and struts and frets around for an hour, and then vanishes to never be heard again."
A big idea, summed up in an image.
One important thing is that this isn't a one or two-word metaphor ("Life is just an act!")—Shakespeare builds a little scene, with a stage and an actor and passage of time.
That's something we can experiment with—to what degree can we develop our a metaphor in our variation?