Casting from your phone to your beamer allows you to display your phone screen on your beamer. You can simply cast your favourite app without first installing it on the beamer and logging in.
For example, you can use screen mirroring to show a presentation during a meeting or even during class. This can be very useful if your presentation is already saved on your phone and you just simply have to connect your phone. No need for bringing a laptop (you do need to be sure the beamer supports screen mirroring, however). You're also able to share a holiday destination pitch for your family and friends, or even watch holiday photos and videos, showing your adventure.
Casting your favourite apps to the beamer can be useful if you do not prefer first installing the app and logging in. You might be in a hurry or just prefer this way of casting your favourite movies or series; both are entirely possible. Instead of everyone looking at a small phone screen, the content can be viewed together on a large projection.
However, casting from your phone does not always work with every app. There are some streaming apps/services that have built-in security or content protection which might prevent screen mirroring. This is not a defective beamer or app; it's just content protection against piracy. When this happens, you might get a black screen but can hear sound.
A black screen does not always mean that your beamer or phone is defective. It may just be the cause of the security of the app/service. So don't be scared it's broken! Installing the app directly on the beamer fixes this problem easily. Simply open the app on the beamer, log in and stream.