It used to be that you could leave an app open on your Roku and resume where you left off when you returned. Say you're watching a long YouTube video and hit the pause button and run to the store. When you got back the video was still paused and you could resume right where you left off.
At some point the functionality changed and now you're brought to the home screen after a period of inactivity.
Frustrated, I asked around in the forums only to be told this was an effort by Roku to force users to see ads displayed on the home screen. I was originally skeptical of the claim but now I realize that was Roku's plan all along.
I stopped using my Roku after that move. I've since switched to using the apps on the LG TV which don't "auto-stop" after a period of inactivity.
"Pay no attention to the man behind the curtain." -- The Wizard Of Oz