And yes you DO need to learn all this stuff even if you dont use it at all
Nearly all the ATPL knowledge is either required or useful later on. OK maybe no grid navigation, but pretty much everything else.
Apparent drift of gyros? WTF?-it's only apparent if you are a stationary observer on the Earth's surface. Why the hell is this essential knowledge for a pilot?
Because without knowing it one wouldn't understand how to navigate with a DI or with a failed IRS in attitude mode.