You not only want to dismantle and make it work again, you rather take it apart in an effort to rebuild it using your own parts. So from a finished product you in the end really understand how it works and can replicate that. Sometimes this requires you to destroy the original product. (Btw: you can also reverse engineer code for example. Not only hardware)

It’s figuring out how something works out or is manufactured from the finished product, possibly including dismantling it to do so.

i don't say this often, but you could've literally searched on wikipedia..

People do it for electronics all the time on YouTube, see bigclive etc