A stereo power amplifier is most often used in conjunction with a preamplifier or surround processor, to drive larger speakers or if you simply want the best control and thus also sound quality. A stereo power amplifier is also useful in a bi-amping setup, where you use 2 stereo power amplifiers - one for each speaker.
Monoblocks are often used in connection with Hi-end stereo systems, where you want a dedicated mono power amplifier for each floor speaker, for maximum power/control and sound quality.
In professional installations, passive subwoofers are often used for higher flexibility and easier cabling, i.e. it does not require power where the sub is placed/mounted. For
passive or built-in subwoofers it is therefore necessary to use a power amplifier to drive the subwoofer, as there is no amplifier built into a passive version. You can of course also use a
stereo and
multichannel power amplifier, e.g. the DSP models, if you need an amplifier for multiple subwoofers or simply more power by means of bridge connection, etc.
Always remember that there must be a low-pass crossover filter built into the amplifier, or an active filter before the power amplifier, if it is to be used as a subwoofer. This can of course also be in the form of the subwoofer output of a surround receiver/processor or a DSP crossover filter set up in a
professional power amplifier.