It would seem that hearing-impaired people are not the only ones who benefit from captions. A San Francisco State professor of American Indian studies just announced that he saw enormous changes in comprehension if he used captions on videos shown in class. While he focuses on the impact his observation can make for Native Americans students, I don't see any reason why his observation would not carry over to all students (well, those who can read :) ).
To quote from a SUNY Cortland website on learning modalities:
Learning modalities are the sensory channels or pathways through which individuals give, receive, and store information. Perception, memory, and sensation comprise the concept of modality. The modalities or senses include visual, auditory, tactile/kinesthetic, smell, and taste. Researchers, including Reiff, Eisler, Barbe, and Stronck have concluded that in a classroom, the students would be approximately:§ 25-30% visual
§ 25-30% auditory
§ 15% tactile/kinesthetic
§ 25-30% mixed modalities
Therefore, only 30% of the students will remember most of what is said in a classroom lecture and another 30% will remember primarily what is seen.
While videos are nominally visual, the facts they are intended to convey are often presented via spoken words. If the learning modalities theory is accurate, 25-30% of students watching a video may learn a lot from the moving pictures but will have difficulty remembering what was spoken. By turning on captions on a video, that 25-30% of students (and probably many of the "mixed modalities" students, too) will be helped to better remember the information presented.
I think my children are too young to appreciate captions right now, but when they become faster at reading and can easily read subtitles while following on-screen action, I will turn on the captions for the educational videos I show them. Maybe I'll even do some experiments - captions for one child, no captions for the other, and the same comprehension quiz afterward. The home is a social science laboratory, after all.