Collecting something, having a binder with your collection, showing it to people... is an attractive activity for many, and may not be that different from playing against said people.
I was deep into MtG (playing in tournaments, wining a few small ones, throwing a few hundred bucks at it), although I've never gotten into any other collection hobby.
To me, playing was the best part and what gave meaning to the rest of it. I loved the competitiveness, and coming up with strategies... but also the social aspects of it, of seeing others do the same activities. Many of these social aspects can still be there for "collecting without playing"-alternatives right? I enjoyed greatly seeing rare cards in other peoples collection, and getting a few of my own (rare editions, customized units, misprints...)
So yeah, for people that only care about the competitive aspects, MTG is totally different from baseball cards; for others, both hobbies may share many aspects.