My gas burning heating unit takes its temperature readings from a 433 MHz thermostat, which is emitting precisely the same data packets in a packet train at some ~100 ms interval like this: 2 x OOK and 4 x FSK. The trains are emitted once a minute. What does this technique have to gain as opposed to emitting only in FSK ?
Probably ease of detection, or using the known OOK signal as kind of a frequency-synchronization and rate-synchronization method.
It's hard to tell – in low-power and least-cost electronics, the emphasis often is not on RF signal optimality, but in ease of production and power usage, things you won't find out without knowing the silicon design process.
It could also simply be a compatibility thing: maybe that thermostat is supposed to work with two different systems, one of which understands OOK, the other FSK.