Games : What the impact of 'co-belligerent' check-box?

When I declare war on country that has allies I have option to chose any of those allies to be considered as co-belligerent.

If I don't check than I will suffer penalties trying to get army from those countries. But what if I do? I don't see any drawback to have that check-box always checked.

Do I miss something important?

Thanks.