An icon can be any picture even any type of application can have our own image as an icon. But though the software allows us to keep any picture as an icon, it is recommended to keep the icon so that it depicts what actually the software is.
As given in the question the icon in YT player tells us that, the application has a capability to play video format content which can be viewed both offline and online. Most of the video software has “play” symbol as the icon.
