PowerShell string default parameter value does not work as expected

Question

Okay, found the answer @ https://www.codykonior.com/2013/10/17/checking-for-null-in-powershell/

Assuming:

Param(
  [string] $stringParam = $null
)

And the parameter was not specified (is using default value):

# will NOT work
if ($null -eq $stringParam)
{
}

# WILL work:
if ($stringParam -eq "" -and $stringParam -eq [String]::Empty)
{
}

Alternatively, you can specify a special null type:

Param(
  [string] $stringParam = [System.Management.Automation.Language.NullString]::Value
)

In which case the $null -eq $stringParam will work as expected.

Weird!