If you don't do this, you could make the first exposure on the fogged portion of the leader and not get the picture. With an auto-loading camera, line the end of the film leader up to an index mark along the bottom rail. Check that the advance gear engages the film perforations and that the film lies flat. Close the back and press the shutter release. The camera will advance the film to the first frame.
Extra film is included for a leader at the beginning of the roll and for a trailer at the end.
The processing laboratory needs the leader as well as the trailer when processing the film. Electrical contacts in the film chamber enable many cameras to read DX-encoded films to automatically set film speed, determine the number of exposures on a roll, and the exposure latitude of the film. With a manual-loading camera, you can use the rewind knob to check the film. Turn the rewind knob carefully in the direction of the rewind arrow until you feel a slight tension. This takes up the slack in the film.