Why are secured services added to ArcGIS Online with stored credentials remapped to utility.arcgis.com?

Answer

Secured web services provided by ArcGIS Server and hosted services may be added as items to www.arcgis.com.

If the option to store user login credentials is selected when adding an item, the item URL is updated to reference utility.arcgis.com. The URL changes to the following format:

https://utility.arcgis.com/usrsvcs/rest/services//MapServer.

Note:
When the Do not store credentials with service item. Prompt for authentication every time option is selected, the URL remains the same as the service.

When secured service credentials are stored with the item on www.arcgis.com, a proxy application is leveraged on the utility.arcgis.com service URL. This proxy assists with dynamically generating a short-lived token to allow user access to the secured resource without the need to interactively pass user credentials.