why a subinterface on cisco router

hello expert,

I am struggling to understand the concept of subinterface on a router.

we recently had a local isp configure our new cisco 1841 router for frame-relay connect to our head office

router.

The local phone company has provided one PVC, because this is suboffice to head office connection.

From the configuration below i notice the tech has configured a subinterface and assign the serial ip

address to this interface.

when i ask him to explain why configure a subinterface when the physical serial interface could

have been use for the configuration.

His responsed, he use the graphical wizard to do the configuration and during the process was prompted to

configure a subinterface.

I do think there is no justification for a subinterface, since there is one one PVC for this

particular connection ( rosignol to head office).

I am total confuse about this entire  concept of subinterface.

can any expert review the above and provide suggestions

I have attach below a the suboffice router running configuration.

Thanks





Rosignol RUNNING-CONFIGURATION




Current configuration : 2433 bytes


!
version 12.4
no service pad
service tcp-keepalives-in
service tcp-keepalives-out
service timestamps debug datetime msec localtime show-timezone
service timestamps log datetime msec localtime show-timezone
service password-encryption
service sequence-numbers
!
hostname Rosignol
!
boot-start-marker
boot-end-marker
!
security authentication failure rate 3 log
security passwords min-length 6
logging buffered 51200 debugging
logging console critical

!
no aaa new-model
!
resource policy
!
clock timezone PCTime -4
mmi polling-interval 60
no mmi auto-configure
no mmi pvc
mmi snmp-timeout 180
ip subnet-zero
no ip source-route
ip cef
!
!
ip tcp synwait-time 10
no ip dhcp use vrf connected
!
!
no ip bootp server
ip domain name yourdomain.com
!

!
!
!
interface FastEthernet0/0
 description $ETH-SW-LAUNCH$$INTF-INFO-FE 0$$ES_LAN$$FW_INSIDE$$ETH-LAN$
 ip address 208.130.5.3 255.255.255.0
 ip helper-address 208.130.1.2
 no ip redirects
 no ip unreachables
 no ip proxy-arp
 ip route-cache flow
 duplex auto
 speed auto
 no mop enabled
!
interface FastEthernet0/1
 no ip address
 no ip redirects
 no ip unreachables
 no ip proxy-arp
 ip route-cache flow
 shutdown
 duplex auto
 speed auto
 no mop enabled
!
interface Serial0/0/0
 no ip address
 no ip redirects
 no ip unreachables
 no ip proxy-arp
 encapsulation frame-relay
 ip route-cache flow
!
>>interface Serial0/0/0.1 point-to-point
>> description Frame Relay to Head Office
>> ip address 208.130.4.4 255.255.255.0
>> no cdp enable
>> frame-relay interface-dlci 16 IETF
!
ip classless
ip route 208.130.1.0 255.255.255.0 208.130.4.1
ip route 208.130.2.0 255.255.255.0 208.130.4.2
ip route 208.130.3.0 255.255.255.0 208.130.4.3
!
ip http server
ip http authentication local
ip http timeout-policy idle 60 life 86400 requests 10000
!
logging trap debugging
no cdp run
!
control-plane
!
banner login ^CAuthorized access only!
 Disconnect IMMEDIATELY if you are not an authorized user!^C
!
line con 0
 login local
 transport output telnet
line aux 0
 timeout login response 15
 login local
 modem Dialin
 transport input telnet
 transport output telnet
 stopbits 1
 speed 38400
 flowcontrol hardware
line vty 0 4
 privilege level 15
 login local
 transport input telnet
line vty 5 15
 privilege level 15
 login local
 transport input telnet
!
scheduler allocate 4000 1000
end

Rosignol#