Why do some tutorials use Main.cpp & Component.h versus Audio App Template...?

I am kind of new to JUCE and trying to wrap my head around the framework for making some I/O type of apps.

Why do the tutorials like ‘ComponentsParentsChildrenTutorial’ and ‘LoopingAudioSampleBuffer’ use only a Main.cpp and a .h file where as when creating a new Audio Application from the Projucer templates, the default setting is to create a Main.cpp + .h/.cpp…then there’s even a discrepancy in the 'Tutorial:Projucer Part 3: Choosing the right Projucer template for your application ’ where it explains that the Audio App template will include a Main.cpp and MainComponent.cpp.

Is there any rhyme or reason that I can pick up on from this difference for developing my own apps? Is there a convention for when to merge header and implementation into a single .cpp or .h?

Any insight would be great! Thanks