When you are working on a phone, you don’t need to work on multiple Android apps together. You are working on only one android app. You don’t split app windows to see the another so when you switch to another app, background apps are not noticeable, that is why they are paused.
Now, when you switch to another app, the app you select to see is running and other went pause. But, same case should not happen when you are working on laptop because on laptops sometimes user work on multiple windows together.
Chromebook is the laptop running on linux based Chrome OS and Google recently enabled Chromebook for running Android apps on it but there are some limitations. Now, major limitation of parallel app is going to be fixed with Chrome OS 64 and you can run multiple android apps all together.
If we talk about Chromebook and Android phone, both are different things. Phone is used for limited usage. So it is not important to work on multiple applications simultaneously. But in case of Chromebook user want to work on multiple windows simultaneously. If he he is playing a game and wanting to look at the Google analytics app both should be in running state. Switching focus on analytics should not pause Game and vice versa.
Chrome OS 64 is available in beta channel and the news was first broke by ChromeUnboxed and shared a video demonstrating the functionality.