“There’s several general motivations — one is that it’s a challenge,” says Russell Belk, professor of marketing at York University in Toronto. “But it’s a challenge within a smaller world than generally succeeding in the world of business or a career, and so there’s a greater chance of success.”

“I expect no matter how beautiful or ugly the Pokémon is, there’s relatively little aesthetic judgment,” says Belk, an expert in the psychology of collecting. “You want them all — or as many as possible.”

Collecting Pokémon is a lot like building a coin or stamp collection. It involves taxonomy — the process of naming and classifying things into groups.

Belk adds that the desire to collect isn’t driven by a need to complete a collection. “You’re not striving for that closure as much as striving for bigger and better collections,” he says. “That implies some social comparisons — that your collection is in some sense better than theirs.”

Collecting digital objects can have advantages over physical possessions. While coins and stamps are kept in cabinets at home, you can store an entire collection of Pokémon on your phone to show friends.

One reason why Pokémon GO is so popular is that it puts digital monsters in the real world. Like finding a rare book in an antique shop, this turns the discovery of Pokémon — the challenge or thrill of the chase — into a story.

“With augmented reality, they’ve made the ‘thrill of the hunt’ in a version where you can tweet about it, you can post about it on your website, you can carry around images of the Pokémon that you’ve collected,” says Belk. “That’s a conversation piece, and something you can carry with you or brag about online.”

“Digital space in the cloud or on your computer is almost limitless at this point,” he says. “It’s estimated that about one out of three people collect something, and if you include things like music and photos, it’s much higher than that.”

Belk thinks collecting can even be good for you. This is especially true if you’re unemployed, as it lets you set goals and judge your own achievements — that even includes catching Pokémon.

“We sometimes see collecting as fairly frivolous, but it can provide real benefits.”