They are an important part of the brick or masonry veneer system. In building construction, weeps are typically found in a masonry veneer or cavity wall, just above the flashing. The cavity serves as a way to drain this water back out through the weep holes. The weep holes allow wind to create an air stream through the cavity. The stream removes evaporated water from the cavity to the outside. Weep holes are also placed above windows to prevent dry/moisture rot of a wooden window frame.
Now the other day while doing an inspection on a 5400 square foot six year old home, I found something that I see a few times a year. A homeowner thought that the weep holes were a defect or mistake made by the mason when the brick was placed on the home. So Mr. Home Owner decided that he needed to seal each and every single weep hole with silicone.. I stopped counting at the 26th sealed weep! This really could be a major problem if the silicone can't be fully removed from the weep holes. It would defeat the reason for the weeps and could cause major damage to the wall of the home.
So never seal those open holes in the brick walls on your home!