Since the OP mentioned iPhones, if Apple’s HomeKit presence is reliable and smartthings presence is not, you can also use a physical device visible to both platforms as a proxy, which is what I do.
How to Use a Meross Plug to Use HomeKit Presence to Trigger ST Routines
The point is you might want to have a Virtual presence sensor just as a Troubleshooting Device, so you can easily see what smartthings thinks your current presence is based on its own calculations.
Or a lot of people will use a virtual presence sensor as a proxy so they can take the presence state from a different platform, as @Terri_Baker mentioned, that into smartthings, so they can then trigger routines off it. In that case you might even bypass smartthings presence all together and just turn the virtual device on from the other platform.
So it just depends on why you want to have it. Do you want a virtual presence sensor as a visual indication of what smartthings thinks presence is in this moment based on its own algorithm?
Or do you want a virtual presence sensor so you can let another platform control the state changes based on that platform’s algorithm?