The way that I use it is to put a 0 (in your case 9) in front of the incoming number so that when a user dials from their call history it doesn't have to be modified for the call to go out.
The other use for it is to strip an extra digit off of the CLI display as when the call comes in via SIP it is delivered in the 881231234 format, where the extra 8 at the start isn't needed, so it gets stripped off and presented to the phone as 81231234 which is a local number. The other thing to do would be to get it to insert an extra 0 and leave the extra 8 then the number would be a full national number, but users would rather see the local number version for a local call.