Often times betting comes down to situation and number, and watching can sometimes complicate that.
Our eyes are easily fooled that's why knowing the numbers needed become important.
There are many, many downfalls to watching, including treating it differently if you are betting and watching.
It can be good to watch if you're doing it in the context of trying to see what the game, teams, or sports are showing others. You can be a consumer to consume and be entertained, or you can be a consumer to analyze what everyone else is consuming and see how they are reacting to it.
Always good to at least think about what it takes to be a step ahead of the rest of the marketplace.