An ozonator provides supplemental oxidation to get rid of bather waste and also to kill some pathogens faster (but they have to find their way to the ozonator via circulation and cannot be stuck on spa surfaces). If you use the spa every day, then having an ozonator may cut your chlorine usage roughly in half (if you are using bromine instead, it will cut that usage down as well).
On the other hand, if you don't use the spa frequently, then an ozonator will increase chlorine demand, perhaps doubling it (either from oxidizing chlorine to chlorate or having faster chlorine outgassing) so instead of losing 25% of FC each day when the spa is not in use you would lose 50% each day with an ozonator.