Comedians spend years performing in rooms far smaller than the ones they could fill. The practice is not humility, it is the only reliable method of finding out whether material works.
A joke cannot be evaluated on paper
Written down, a joke has no timing, no delivery and no audience, and those are the components that determine whether it lands.
Laughter is also involuntary and immediate, which makes it a rare form of feedback that cannot be faked to be polite.
The room supplies information available nowhere else: exactly which word produces the response, and how long the response lasts.
Small rooms allow failure
New material fails frequently, and failing in front of forty people costs almost nothing.
Failing in front of a large audience that paid substantial money for a ticket is a different proposition, both for the audience and for the performer's confidence.
Clubs and open spots function as a laboratory precisely because the stakes are low enough to try something that might not work.
Room size also changes the sound of the response. Forty people laughing is audible as individuals, which tells a performer how many are laughing rather than only that laughter occurred.
The audience is not there for you
In a small club, most of the audience came for an evening rather than for a specific comedian, which removes the goodwill a fan brings.
Material that works on an indifferent room will work anywhere. Material that only works on an audience already inclined to laugh has not been tested.
This is why comedians preparing a tour will deliberately play unadvertised sets, appearing on a bill without their name attached.
Structure is found through repetition
A premise is performed dozens of times, and each performance reveals which setup is unnecessary and which detail is doing the work.
Material generally gets shorter through this process. Words are removed until only the ones producing laughter remain.
Tags, the additional punchlines attached to an established joke, are discovered live rather than written, often arising from something said accidentally.
Comedians record their own sets for this reason, since the memory of a performance is unreliable and the recording preserves the exact phrasing that worked.
Why the finished set sounds spontaneous
An hour that appears conversational is usually the product of a year of these small performances, arranged into an order that has itself been tested.
The apparent looseness is a construction, and the pauses are placed as deliberately as the words.
Recording that hour ends the process, which is why comedians retire material after a special and start again from empty rooms.