Taking the Splatoon example, spending $25 and getting Off the Hook amiibos is satisfying. I paid money and I actually a physical thing that has weight that I can physically put somewhere in my home. Whereas if I paid 5 cents on an in-game crown, I'd probably be like "huh? Did I really just spend 5 cents on that stupid crown?"
It's a psychological thing. Exclusive clothing packs in Splatoon are essentially being sold, just in amiibo form and not in-game. Psychologically though, you're buying a toy for whatever price and getting an in-game costume as an extra, instead of just spending money on a costume