well there are a number of reasons.
First of all, simply watchig videos, a lot of the time, you dont even understand why a Pro did a certain move in a certain situation as opposed to using some other strategy. Especially when you're starting out, watching really high level vids can only help insofar as you already understand 100% of whats happening. Newer players might not even understand why (for example) mango isnt being sheild grabbed when he fades away with nair while pressuring.
Secondly, it can take hours (or be flatout impossible) to find a video to cover a specific situation you're having trouble with. Its impractical to search through the thousands of hours of youtube vids to find a solution to something you can just ask about here.
And finally, its entertaining to discuss things with other people who share common interests. :/