Academic claptrap. isnt it time we got rid off all this nonsence?
I am with you on this one. Know the compass has errors, know the mnemonic to remember them and you are done.
If you want stuff to study get into the POH. You should be
very familiar with the systems descriptions and the amplified procedures in the emergency section'
Read the operators manual provided by the engine manufacturer and understand how the major accessories on our engine work (eg mags, starter, alternator, fuel pump, vacuum pump) with an emphasis on knowing what symptoms they will show when they are not on their way out.
Here is a skill testing question in a subject way more useful than knowing the theory of why the magnetic compass has errors. You are doing your run up and there is no appreciable drop when you select the left mag and a 50 RPM drop when you select the right mag. Is this OK, and if not why not ?