For incoming calls to a gateway, an inbound dial peer is necessary. That being said, if a configured inbound dial peer cannot be matched, the gateway will use the default dial peer. This is sometimes not desirable if the call leg is using non-default codec, capabilities, etc. It is a recommended best practice to explicitly define your inbound dial peer. Take a look at the following link for further explanation.
http://www.cisco.com/en/US/customer/tech/tk652/tk90/technologies_tech_note09186a0080147524.shtml
Hope this helps. If so, please rate the post.
Brandon