Silent films were inconsistent on frame rate, especially early on. The cameras were hand cranked and so the frame rate was determined by the operator. It wasn't unusual to speed up and slow down the frame rate within a scene to achieve a desired effect. The person cranking the projector, in the early days, had to make a similar judgment. As hand cranked projectors have way to electrically powered ones, handling abnormal speeds became difficult, which lead to standardization. Early silent films almost always run too fast when projected on modern projectors. As a result, variable speed has always been a desirable feature for projectors used for historic films. Film preservationists would often modify projectors to provide it.

It can similarly be very difficult to find aperture masks to fit the unusual aspect ratios of older films, so a projectionist might have to fabricate one. Fortunately that's pretty easy with a file.

Some films were recorded with a lower frame rate due to some reason. As for variable and standstill, I think this would only be needed when investigating a recording in detail. Not sure if they really need it. Perhaps they just want it to cover more use cases.

Silent films used several different speeds. The rest are for telecines.