It may be that you don't want everyone to be able to dial a particular overlay. Perhaps a user is paying for calls to an destination, or perhaps a number is for a busy member of staff who should not be bothered by every call. By restricting the dialing number allowed to make a call, you can lock down who is allowed to use an overlay.
A customer needs the phone number of a senior member of staff, you do not want this number to become generally available. It would be possible to create an overlay and use the expiry options to ensure that it is not valid for long, but that would still allow calls through in the short term, additionally, the customer may want to use this number for a long time. Fortunately, you know what number the customer will call from, so it is possible to reject calls that do not come from that source. To do this, provide the 'cli' parameter in addition to any other requirements.