From a guitarist point of view, in a class A amplifier each of the amplifiers power tubes are working at maximum output the whole time. This makes them run ‘HOT’ and generates a very natural level of smooth tube compression.
FX loops are important as they allow you to add effects to the signal path in a specific place between the pre-amp and the output section on an amplifier. This is where FX loops are important as it allows you to put time-based FX after the distorted pre-amp section. For example, delayed distortion sounds different to distorted delay – try it.