It adds to the cohesive narrative which can make the overall story telling experience better. When works are considered canon, that's usually an indicator that the overall franchise narrative is being cared for and thought of.
It can also mean that some aspects of one show can show up in another or vice versa. It's very cool to see a character you really like. Show up in another medium.
For your second point, you can do that without worrying about canon. In fact it's easier if you don't.
For the first one, that can be interesting for a single coherent work with a single writer or team of writers that planned out at least an outline of a story beforehand, but there are rarely questions about canon for those kinds of stories. For something worked on for over half a century by hundreds if not thousands of people many of whom have very different visions for what it should all be about, in my experience it just leads to people getting mad at each other based on which of those visions they like more, and lots of self-contradictory nonsense crammed into the story to try and make it all work. That is just my opinion though. You are free to like different things than I do.
I've never personally understood why people get so obsessed with which works of fiction are more real. It's just a silly thing to worry about.
It adds to the cohesive narrative which can make the overall story telling experience better. When works are considered canon, that's usually an indicator that the overall franchise narrative is being cared for and thought of.
It can also mean that some aspects of one show can show up in another or vice versa. It's very cool to see a character you really like. Show up in another medium.
For your second point, you can do that without worrying about canon. In fact it's easier if you don't.
For the first one, that can be interesting for a single coherent work with a single writer or team of writers that planned out at least an outline of a story beforehand, but there are rarely questions about canon for those kinds of stories. For something worked on for over half a century by hundreds if not thousands of people many of whom have very different visions for what it should all be about, in my experience it just leads to people getting mad at each other based on which of those visions they like more, and lots of self-contradictory nonsense crammed into the story to try and make it all work. That is just my opinion though. You are free to like different things than I do.
I just see canon as a toolbox for building one's personal fanon/headcanon from.