ValueFromPipeline.md

The whole point of using ValueFromPipeline (and ValueFromPipelineByPropertyName) is to replace this syntax:

"c:\windows","c:\Program Files" | ForEach-Object {
  Get-DirectoryFileSize -Directory $_
}

With this synax:

"c:\windows","c:\Program Files" | Get-DirectoryFileSize

Even when your function/cmdlet only supports operating on a single thing at a time, when you declare ValueFromPipeline/ValueFromPipelineByPropertyName on a property you need to include the process block so that a user can send a collection that PowerShell will unwrap for you and your function will still work.

Without a process block, the "Step 2" version of the function will only operate on the LAST item in the pipeline, which is not expected behavior.

"c:\windows","c:\program files" | Get-DirectoryFileSize
  
Directory         SizeInMB    
---------         --------
c:\program files         0