Well, if a Yoga student is performing Sun Salutations and is healthy or focussed, why should he/she study Yoga philosophy and get headache? Good question.
The main reason is: Yoga is a Philosophy ONLY, in the originality. It is called Samkhya Yoga in the Classical Terminology. It is a branch of Indian Philosophy, called Darshanas. Thus Yoga is Yoga-Darshana.
In order to align with that philosophy, a practice has been developed and that is called Yoga Practice. And, therefore, if one wants to call their practice Classical yoga practice, they must know what Yoga Philosophy says and check out whether their practice is aligned with that practice. Otherwise, they should not call their Yoga practice Classical one. They may still call it whatever they like, even otherwise.
Thirdly, some very serious Yoga students of today are wondering why they are not getting effects mentioned in Classical Yoga circle or ancient texts. Where are all those yogis who go into Samadhi? Why so many world famous great yoga teachers of today are on the stage but not in the Samadhi? ETC. This is because, they are not doing the Samadhi practice. Thus, today’s serious Yoga students get knowledge that helps them to update or upgrade their practice.
Yes, Yoga is a practice after all. But, practice does not mean any practice such as legal practice or gymnastics practice. Whatever your practice is will give results accordingly. Thus, one should know what their practice aligns with, whether with fanaticism, with limitation, with gymnastics, with mysticism, with commercialism, or with egoism.