Taking things apart can teach you engineering, assembly techniques, and repair methodologies.
I started doing teardowns when I was 8 years old. I would take apart flashlights when my parents were out, only to not be able to put them back together. I had better luck with ball-point pens, although the little spring would get away from me now and then.
Peering inside a product can teach you design engineering as was as manufacturing engineering. Its a great way to learn how things work and how they go together. Its always good to improve your manual dexterity.
Some teardowns are just that-- curiosity as to what is in side. Others are to fix something. Still others are to determine the cost of a product or how it works.
Just like those flashlights and ball-pint pens, taking things apart is a lot easier than putting them together. So a really successful teardown also includes putting the product back together and having it work.
The other aspect of teardowns is responsible environmental stewardship. Web sites like iFixit started because the founders resented they could not replace the batteries in an iPhone. After posting teardowns, they learned they could finance the site by selling tools needed to pry apart a modern gizmo.