Sometimes is handy, but sometimes you want that app to be full screen, the using the dock opens it in sidebar if it was previously like that, but, what if I need it in full screen?. So far I need to go to home and open it from there or move to the app switcher look for the app and select it.

I think it would be a better behavior if selecting he app from the dock opens it in full screen.

What if I want an app from the dock to be in the sidebar?

You already have an option to do that. And if that app is a ready in the sidebar you can just pull from the right side of the screen and it will show up.