I'm not a programming professional, but as I understand it: you study for interviews because the biggest tech companies used to ask difficult data structure & algorithms-based questions, plus some difficult/tricky questions which require a particular style of thinking. Now nearly everyone does it, and YouTube and textbooks offer lots of help... Which has started an 'arms race' in knowledge about DS&A and answers to tricky problems between interviewers and interviewees.

At some point I think it was intended to be like a 'coding-oriented IQ test', after you'd passed a set of HR checkboxes. Obviously an IQ test you study carefully for is not a broadly applicable measure of relative performance. As far as I understand, many of the top companies are shifting away from this interview paradigm, but it'll take the rest of the industry which adopted it a long time to catch up (I imagine).