I don't know. Why did you or your organization create a sub-interface?
If your outside interface needs to support more than one VLAN then you have to create sub-interfaces for those VLANs and apply policies to them...Maybe you want TCP port 80 allowed through on one VLAN but not on the other...
The question "why" can only be answered by knowing your setup and your requirements. If you figure those out you'll figure out why...
I have looked into this a bit more and the only reason I can see that this is done is so you can easily move the config of the interface to another interface.