Photo from Sherwood and Lloyd Schwartz' "Brady, Brady, Brady"
In general, stand-ins are used on every set to literally stand in front of the camera while the crew sets the lights and the camera shots. They need the actors in place to frame and light them for the camera. By substituting stand-ins who are similar build and height, the actors are free to do other things, take a break, or in The Brady Kids case, they could be in school while the lights and camera were set. Rather than hire children for this, Frank and Sadie were adults, and the right size, so they could work longer hours, and did not need guardians and teachers.
At some point the kids outgrew them and, I suppose, they would stand on apple boxes, a common film set item to stand on, or maybe they both just worked on the show less.
