All anime has soundtracks for sound effects, bring the mood to certain events, openings, and endings. These are either very popular j-pops (Japanese Popular Songs) or j-pops that become popular because of the anime. This is exactly the same way movies have famous tracks or make famous tracks! For example, 'Mr. Blue Sky' becoming well-known because of Guardians of the Galaxy. Another example would be 'Bad Guys' from Billie Eilish being famous and put into movies like "The Bad Guys", "Sing 2", and "Back to the Outback".