That's true. But:
OST - Original Sound Track and they usually include the openings and endings on the albuns, hence my previous reply.
Also, pretty much the animes whose openings I posted have super duper soundtracks (Yeah, DanganRonpa Yeah )
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Yes they are included in the ablums but some openings arent just for the anime.
This is to aru opening but its also in other ablums such as the original artist
https://www.youtube.com/watch?v=OJT8C6ja5ts
OST covers the openings and endings, you'll find them on most every one of them. Though yes, the authors do make music videos of their most popular songs and the anime only helps to boost that.
https://www.youtube.com/watch?v=AC6REhy5L38
-Black lagoons opening.
CLANNAD OST
https://www.youtube.com/watch?v=5KMzDVf2Bx4
its an op but i love it; Phantom|Requiem for a Phantom https://www.youtube.com/watch?v=OkmMsH5ssc8
Old but Gold; Naruto https://www.youtube.com/watch?v=wEWF2xh5E8s
Great anime, CodeGeass; https://www.youtube.com/watch?v=TWn0nF-PFcE
Pandora Hearts; https://www.youtube.com/watch?v=UQkxNbgohPg
KHR; https://www.youtube.com/watch?v=qq9dhnApETI
and a lot moreeeeeeeeee