I think a large part, as others have said, is to collect physical media, but another large reason could be because people want to support the artists they listen to by actually buying their music. Streaming services are the most popular way to listen to music now and they are so fucking awful when it comes to compensating artists that most of them cannot even come close to relying on them for a stable income, even when you take in to consideration streaming on multiple platforms. So doing things like buying a record or a CD or even just buying an album digitally on a site like bandcamp does a lot more for the artists that you enjoy because they receive a much larger fraction of that money. Also, doing things like buying merch and going to shows are other good ways of supporting the artists you love!