Here's an image of the take-up spool from the Canon SII manual (Canon rangefinder c. 1940's) The spool has a metal clip to securely hold the film leader to the reel.
This configuration is then loaded from the bottom of the camera, like so:
The clip, not pictured, is a piece of V-shaped metal that holds onto the leader by wedging the leader up into it. It's a decently secure connection that would be more difficult to make if the V had to travel the entire length of the reel.
Not having this connection makes loading the camera impossible or wasteful (requiring you to spool a few frames around the reel to get it to "stick")
Simply, the leader is a holdover from early 35mm rangefinder designs. For those of us that still shoot these cameras, I'm grateful the market hasn't gotten rid of the leader.