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.