Usually the only way to get halaa marks is to kill someone within halaa range.
How ever as a priest if you mind vision someone getting killed outside of halaa while your within halaa (hope that made sense) you can still get the mark.
I guess this could be useful if say a priest needs halaa marks. And the rest of the group just came along to gank people. As long as your in the group they can kill someone any where within mindvision range and you can get your halaa mark.