If you're into photography and have bought a single lens reflex camera, you'll soon start swearing over the fact that there they don't use the same lens mount.
Canon, Nikon, Sony et al all use their own lens mounts meaning you're stuck with one brand that might have great optics, but only so-and-so cameras. Unless you sell the lot and start again, which is an expensive proposition.
There might be a really great lens from manufacturer A that you would like to use on camera B, but it's not going to work, not usually. There have been adapters around for a few years now, but they tend work with just some lenses, or provide partial features for these. Autofocusing accuracy and speed can get compromised severely using adapters for instance.
Having tried the Metabones III (not the newer IV) adapter with very mixed results to mount Canon EF lenses on Sony A7 cameras, I wasn't sure what to expect from the new Sigma MC-11 device.
This little device sits between the lens and the camera body; the MC-11 is officially aimed at customers who have bought Sigma lenses with Canon EF mount and who wish to use them on their Sony A7 full-frame and other cameras like the A6300 with the smaller APS-C sensor.