I never got the idea of being a hobby collector in modern times. Nowadays, everything is available on the Internet for sale, so completing the collection is only a matter of coughing up enough money. How is that interesting or exciting?
I can still understand collecting items which aren't mass produced for the purpose of being collected - things such as history/war memorabilia, rare books fro XIX century etc. There, every item can be unique and obtaining them can demand more from the collector than just typing in their credit card numbers.
The one thing I can truly say I "collect" is rare, first/early edition pre-standardization works from a specific Irish dialect. For me, though, it's not so much the collecting as having access to these books for the rich, natural (.i. non-standard) language that's contained in them. Eventually I might extend it to modern authors from the same dialect, but getting access to these older works is top priority.
I know some people in the actual hobby collection scene for certain trading cards (as opposed to people calling themselves collectors but actually want to just flip for profit). The items these people collect are things such as rare misprints (with population sizes of below 100), physical printsheets (which sometimes have a population size of 1) or promo cards given out for very specific events (such as the creators wedding), again with single- or double digit population sizes.
From what I gather the appeal is partly just seeing weird things from the history of a hobby you enjoy. It‘s also a kinda nice way to connect with other people in the collection hobby, since when you run into something wacky you can just call up Jon who collects that exact oddity and bring it to him in exchange for a nice dinner or something.
I have cards for certain players, teams or inserts that I buy to collect, if I can make money on them great, if not I enjoy having them in my personal collection (PC).
Then there are cards I’ll buy anticipating a rise in value with the intention to sell in 6-12 months or longer. Typically this involves buying rare cards or good players of sports in off seasons when demand dries up, so for example I was buying a lot of big name football cards over the summer when the football news cycle was slow, and recently moved to buying basketball leading into the NBA season.