There's many reasons, here's a few examples:
- See how easy/hard it is to repair it
- See how easy/hard is is to mod it (say, add a bigger battery)
- Understand the internals, see which components are being used and their quality
As an example of the last one, I'm really curious to see a Joycon teardown so we can find out what kind of sticks they used and how likely it is for them to show drift problems in the future.
Not surprising considering that most 3rd party games are: