These choices allow Brothers to remain just out of the picture so that they can more easily do work the Sisters may have more difficulty with in their garb. Despite being known as Guards or Paladins in some houses, Brothers are not considered part of a security detail, though acting as escorts and making sure the Sisters are safe on outings is part of the job.
It should be noted that both nuns and monks are full-fledged members of the order with equal voting rights, time commitment and membership duties. Both take vows, both go through a four-step journey to becoming a full member. They are not above one another or unequal in any way - they simply do different work for the same goal.