Here are some examples.
Kids can learn to sort cards by number, team, player/character, alphabetically, or as my nephew sometimes does it, by shiny cards and non-shiny cards.
Provide them with a few supplies, and kids will quickly figure out that they can use separate binders and boxes for separate projects.
Most of you reading this can probably describe a few of your favorite cards right off the top of your head: the colors, the design, exactly what the player is doing in the image, maybe even the blurb or the cartoon on the back. And I'm sure some of those cards go all the way back to your childhood.
Creating a to-do list and crossing items off that list is a great thing to teach youngsters, whether the list involves cards or not.
Seeing a Project Through to Completion
This relates to the previous example. How many of you remember how you felt when you completed your first full set of trading cards? How determined did you need to be to find those last few cards?
Taking Care of One's Belongings
Some kids take better care of their cards than others. But storing cards in pages, top loaders, and card-friendly boxes instead of just all over the place can teach a child a lot, and help them enjoy their collection.
I don't think my nephew has made any trades with his classmates yet, but ensuring a trade is agreeable on both ends is important, and I'll try to make sure he doesn't learn that lesson the hard way.
Quite a list!
And when that kid collector reaches adulthood, I think you can see how all of these skills will apply to "real world" situations as well.
I know I'm preaching to the choir here. But if you've ever bought cards for your nieces and nephews, or grandkids, or any kid, and the parents respond by giving you the "oh great, now there will be cards all over my house and I'm going to have to buy even more of them" look, just rattle off some of the benefits listed above and maybe those parents will change their tune.