Take a look at this from chemgeek.
Cyanuric Acid (CYA) is not just for protecting chlorine from breakdown from sunlight, but also significantly moderates chlorine's strength. 4 ppm Free Chlorine (FC) with 30 ppm CYA in a hot (104ºF) tub is roughly equivalent to around 0.6 ppm FC with no CYA. If you don't use any CYA, then the chlorine is too strong and will react more quickly with swimsuits, skin, hair, as well as outgas faster and oxidize hot tub covers more quickly and yes, this increases chlorine demand. You want some CYA in the water to act as an active chlorine (hypochlorous acid) buffer so you have plenty in reserve while keeping its strength to reasonable levels. With no CYA, the chlorine is too strong (unless you try and maintain very low chlorine levels, but that is generally hard to do) while with too much CYA the chlorine is too weak and takes too long to oxidize bather waste and can be too weak to kill some pathogens.