Your question is answered - they expect to raise awareness of the issue, to attempt to build a consensus amongst the International Community that rape is a vile act, and should form no part of a weapon of war.
This is an entirely worthwhile initiative. And no, Hague would not be better employed addressing rapes carried out in the UK - We already have laws and domestic initiatives to counter that. He is foreign secretary, and it his job to put the UKs weight, such as it is, not just behind purely economic or poltical initiatives that favour the UK, but behind initiatives that benefit humanity.