ok, I looked into this and DraftApprovalRoles is not used by default, by default users in the edit roles for the page are the approvers of draft submissions and approval results in publishing the draft.
But there is an optional way to have a 3 step process where approval is a separate step from publishing and users in the roles will get notified of submissions.
To enable this optional 3 stage way of using workflow you would add this to user.config:
<add key="Use3LevelContentWorkflow" value="true" />
Then the draft approval roles should appear under security in page settings.
This was something Joe Davis implemented a while ago to meet one of his client's requirements, I have not tested it thoroughly myself.