powershell scripts to attched apppool to existing site

$config = Get-Content Set-DefaultApplicationPoolConfig.xml
$defaultAppPoolName = $config.WebApplications.defaultAppPoolName
 
[Void][System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SharePoint") 
[Void][System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SharePoint.Administration") 
 
$service = [Microsoft.SharePoint.Administration.SPWebService]::ContentService 
 
Write-Host "Checking presence default application pool $defaultAppPoolName..." -NoNewline
 
[Microsoft.SharePoint.Administration.SPApplicationPool] $newAppPool = $service.ApplicationPools[$defaultAppPoolName] 
 
## The Default Application Pool MUST exist!
if($newAppPool -eq $NULL) 
{ 
    Write-Host ""
    Write-Host -ForegroundColor Red "The default application pool '$defaultAppPoolName' does not exist." 
    Write-Host -ForegroundColor Red "Please ensure the application pool '$defaultAppPoolName' is already registered in SharePoint." 
    exit
}
else
{
    Write-Host -ForegroundColor Green "[OK]"
}
 
foreach( $webAppConfig in $config.WebApplications.WebApplication )
{
    $url = $webAppConfig.url
    Write-Host "Updating $url..." -NoNewline 
 
    $webApp = Get-SPWebApplication $url -ErrorAction SilentlyContinue
     
    if( $webApp -eq $null )
    {
        Write-Host -ForegroundColor Yellow " [Web application does not exist]"
        continue
    }
 
    $currentAppPool = $webApp.ApplicationPool
    if($currentAppPool.Name -eq $defaultAppPoolName) 
    { 
        Write-Host -ForegroundColor Green " [Change not needed]"
    }
    else
    {
        $webApp.ApplicationPool = $newAppPool
        $webApp.Update()
        $webApp.ProvisionGlobally()
        Write-Host -ForegroundColor Green " [Done]"
    }
}