I think to some degree it is stating the obvious, i.e. exactly how it benefits your PhD (your research fits the scope of the conference), networking opportunities, as you are in your third year - how it will provide input to your thesis maybe, if there is some benefit to the department, do mention it.. maybe the poster/talk you give includes input from other researchers from your department, i.e. you represent them..., how the conference will widen your horizon/learn new things/get up to date with latest findings etc. hope that helps.