They were designed for those who used 127 film.
Most cameras used 127 to create square negatives or slides (40mm x 40mm) although there were a few cameras that used it to create 40mm x 30mm or 40mm x 60mm negatives.
A 40mm x 40mm slide would fit into a mount that will work in a projector designed for 135 slides - a so called "super" slide. The super slides were great!
There are some high quality 127 cameras out there. The Baby Rollei comes to mind.
127 became much less popular over time. Kodak discontinued film production in 1995. There is some film still being produced.
My first camera took 127 film:
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