Join-Path and Split-Path both fail if they are joining or splitting file paths that use a drive letter that is not on the local machine. Since PowerShell has Remoting as a core feature, it is not uncommon to do some work in a remote session and some work locally. As a result, you may be processing paths that contain drive letters that exist on a remote machine but not on the local system. If you need to use Join-Path or Split-Path on such a path, you will receive an error message that the drive does not exist. This is very limiting for these otherwise highly useful cmdlets.
A scenario where you might want to do something with this: querying a remote machine to get the path for a file, processing it locally to get the containing folder, and then invoking Move-Item to move the file from remote machine 1 to remote machine 2. If you don't have the same drive on the local system, the processing to get the containing folder work fails.