… satisfy certain municipality requirements. Building officials will not crawl through an attic to take depth measurements of the insulation. The best (and sometimes required) practice is to place the rulers, visible from the scuttle (attic access) hole. One in each far corner and one by the access hole.
… save material. You can see if the material is being blown at the proper depth and not over blown.
… provide consumer protection. When installed properly, the homeowner can see if the insulation, they are paying for, is the correct depth. There is no guesswork as to the quality of your job.