Hi there. Before I realized that I can create multiple screens with vertical arangements in just one screen, I thought that I should create as more screens as it needed. Now its not important anymore.
But I want to know, why should we use multiple screens. I searched in the forum, but there is no info about the reason for it. Could you explain or give some examples?
We can use multiple screens when we have different complex operations to do.
The truth is, we don't need to use physical screens at all.
What do you mean? 
Sounds like a question asked by a teacher.
- a developer does not have to use multiple screens; he / she can use a single screen with Scrollable set to true to allow a screen to scroll. This allows you to display a lot of data on a small cell phone screen with a minimum of fuss. The display might look ugly and that might be fixed by displaying some of the data on another Screen or virtual screen.
- App Inventor apps tend to develop problems (sometimes crash) when more than about 10 Screens are used in an app. Each Screen uses up system memory. If the developer does not close screens when exiting them system memory is used up and the app will eventually crash.
- multiple screens might be used for various reasons: avoid clutter, display different types of data, provide a 'log in' area for the user separate from the main Screen , creating a game with different levels, allow the developer to keep track of code applicable to specific functions, make the app easier to use, etc.
Is this what you wanted to know?
I rarely use multiple Screens in my small sample projects.
That said, I would resort to using an extra Screen if I had a substantially different task to be accomplished differing in logic and structure, not just by data, that could be self contained in a Screen for extended use.
Example:
- A maze player
- A maze generator
- A maze cataloger and loader