Why is there a water tube in the staff?

Question

Here is a quote from Dune (book 1):

The man carried Paul’s banner on its staff – the green and black banner with a water tube in the staff – that already was a legend in the land. Half pridefully, Paul thought: I cannot do the simplest thing without its becoming a legend. They will mark how I parted from Chani, how I greet Stilgar – every move I make this day. Live or die, it is a legend. I must not die. Then it will be only legend and nothing to stop the jihad.

Could anyone explain to me why there is a water tube in the staff for Paul's banner? What is it for?

Answer 1

The upright staff (on which his flag hangs) literally contains a water container and a tube from which to drink.

The man brought a hand from beneath his robe, touched Leto’s cheek. Leto felt the jolt of a slapshot and found himself winding downward into a darkness where a green flag waved. It was the green banner of the Atreides with its day and night symbols, its Dune staff which concealed a water tube. He heard the water gurgling as unconsciousness enfolded him. Or was it someone chuckling?

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The symbolism is fairly heavy-handed. Someone loyal to Muad Dib would (metaphorically) always be able to get a drink if they stand under his banner.