It's for propulsion. Since water is non-compressible and air is, the high air pressure inside the bottle will force the water out with more force than air would force air out with. Which hopefully propels the rocket skywards.
HSciO 2013 (R/S) — Astro (1/3) / Chem Lab (8/X) / Fermi (X/5) / MagLev (4/17) / TPS (3/4)
Our imagination is stretched to the utmost, not, as in fiction, to imagine things which are not really there, but just to comprehend those things which are there. ~Richard P Feynman
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