In addition, Kpop groups put a huge amount of effort into their physical albums. Sales of physical albums are extremely high in Korea (Japan I think too) compared to other countries (just my impression, but if anyone has sources I'd be interested).

Albums are usually sold as boxes with a photo book inside and collectible cards, they are almost more like magazines. Some albums come in different editions with different idols featured - it's common for a fan to buy multiple copies of the same album in hopes of getting the edition or cards of their favourite idol (bias). The fans see album purchases and 'streaming' (playing their youtube singles on repeat) as ways to support their groups and increase their reputation.