The can is for catching purpose only. The fluid comes from the res.
As you pull the pedal up for the last time the fluid from the tube loop goes back into the slave cylinder. That must be free of air. If there is no loop, the fluid just runs down into the catcher can. If you pull the clutch up air goes into the system. With the loop the fluid does not run down it stays due to gravity. That leaves an air free part of the tube connecting to the bleeder. When the clutch goes up it sucks the air free fluid back. I don't even usually have to pump the pedal to get function. This works on many automotive clutch systems.

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Chris
84 280sl
82 300d euro