Let's say you invited your friends over your house, and you are listening YouTube. Some songs are loud, others are quieter. From time to time, you have to go to your computer and adjust the volume. Or even worse, some of your mates make it too loud.
So you would install an app from Play Store to solve this issue, but you realize that it works only if both your Smart Phone and PC are on the same network. You have to seek for your WIFI password, but you have never used it on your phone because you have unlimited 4G.
You can download the desktop app and set up remote volume control quickly. You don't even need to download an Android/iOS app.