The leader (albeit in a much longer form than on most currently produced films) is required to correctly load film into a bottom-loaded camera (such as LTM Leica, FED, Zenit-1 etc) so you wouldn't have to align the film cassette, takeup spool and bare film all at the same time, which would be quite clumsy and prone to fail.

Indeed, some cameras do need the leader. Usually they are manually loaded and winded - they use take-up spools with a slot for the leader. Advancing the film is performed by turning the take-up spool, and sprockets alone would not provide grip secure enough. For example, Bronica 135N/135W, Nikkormat EL-W.

When the film has been fully rewound into cassette, retrieving its full-width end would be tricky as you'd have to perfectly align the end inside the cassette to the opening; retrieval of a narrow leader makes this awkward task much easier.

The leader (albeit in a much longer form than on most currently produced films) is also useful for correctly loading film into a bottom-loaded camera (such as LTM Leica, FED, Zenit-1 etc) so you wouldn't have to align the film cassette, take-up spool and bare film all at the same time, which would be quite clumsy and prone to fail.

The leader (albeit in a much longer form than on most currently produced films) is required to correctly load film into a bottom-loaded camera (such as LTM Leica, FED, Zenit-1 etc) so you wouldn't have to align receiving cassette and film all at the same time, which would be quite clumsy and prone to fail.

The leader (albeit in a much longer form than on most currently produced films) is required to correctly load film into a bottom-loaded camera (such as LTM Leica, FED, Zenit-1 etc) so you wouldn't have to align the film cassette, takeup spool and bare film all at the same time, which would be quite clumsy and prone to fail.