Pretty sure it's useless – I'll ashamedly admit that I've never put much effort into figuring it out. In PS v5.1 I get an error when I try to run it:
PS C:\> get-help -Functionality help
get-help : Unable to find type [System.Windows.Forms.Keys].
At line:1 char:1
+ get-help -Functionality help
+ CategoryInfo : InvalidOperation: (System.Windows.Forms.Keys:TypeName) [Get-Help], RuntimeException
+ FullyQualifiedErrorId : TypeNotFound,Microsoft.PowerShell.Commands.GetHelpCommand
I also like that the documentation tells you it accepts wildcard character, and then contradicts itself:
PS C:\> get-help get-help -Parameter functionality
Displays help for items with the specified functionality. Enter the functionality. Wildcard characters are permitted.
This parameter has no effect on displays of conceptual ("About_") help.
Default value None
Accept pipeline input? False
Accept wildcard characters? false
I thought I am the only one who didn't know the usage of this parameter. I was training few candidates on PowerShell and someone asked me what is the use of this parameter during the class and I was stumbled. I told him that I don't know and I will find out and let them know.