I for one am very, very happy BigHit gives them the possibility to release such quality content for free, instead of trying to capitalize on it. It makes it even more stressful for the members, since making it so "official" comes with the added burden of it having to perform well. And when you're a BTS member, that's a fucking tall order.
JK and Tae are stressed enough as it is over it, without the added pressure of worrying about how it sells, how it charts etc.
Moreover - and I'm especially thinking of Tae here - the music he likes to write for himself is different than the solo songs he's had under BTS. Likely because his style is a little more niche, and might not be considered marketable if left as he writes them. This way he doesn't have to struggle between what he'd like to write, and what he thinks would be more agreeable to fans. i just want him to be able to fully express himself without added pressure, and a mixtape is the best outlet for it.