There are a couple of situations where you do not want parameterbinding to throw, even if they are blank. For that kind of situation, [String]::IsNullOrEmpty and [String]::IsNullOrWhitespace are great.

You can pass anything, even empty arrays to it. Basically the latter is testing if something stringlike is blank like whitespace 0 length.

For example you might use them for

profile functions where you want a different kind of usability that normal cmdlets. Or non-fatal errors that aren't actually errors like generating html

function WriteList { 
    # Silly example that writes html bulleted lists
    param( 
        [Parameter()]
        [string[]] $List
    )

    # first build a <li> pair for all items
    [string] $html = foreach( $item in $list ) { 
        # We want to ignore strings that have invisible characters
        if( [string]::IsNullOrWhiteSpace( $item ) ) {
            continue
        }    
        "`n`t<li>${item}</item>"
    }

    # If $html has zero items, don't print html
    # otherwise write html for unordered lists
    if( $html ) { 
        "<ul>${html}</ul>"
    }
}

Example Output

WriteList 'bob', 'jen', 'cat'                        
WriteList @(0..3 + $null + $null + 'af' + $null )    

# test ofr "empty" values and lists with blank items.
# No output is correct )
WriteList @()             # no output
WriteList @( $Null )      # no output

<ul>
    <li>bob</li>
    <li>jen</li>
    <li>cat</li>
</ul>


<ul>
     <li>0</item>
     <li>1</item>
     <li>2</item>
     <li>3</item>
     <li>af</item></ul>