But alternating between full throttle and off is slightly different?
Not really, the heat loss from a hot boiler (IE one full of hot water at 60C, say) is the same whether it is on/off/modulating.
It makes sense to get the maximum output alongside this loss.
A boiler that has modulated to a low output and continued to run is likely to be woefully inefficient in practice, despite what the brochures say.
And you want to ensure that the pump stops operating when the boiler is off (Not all systems do)