The electrical wire entering the bottom of the garbage disposal should be well secured to the garbage disposal for several reasons.
Metal sharp edges
The sharp edges of the disposal hole may damage the insulation on the wire entering the disposal from constant vibration when the disposal is operating. This can result in the disposal becoming energized and an electrical shock could occur. Having a proper connector will prevent this.
Secure wire entering box
Electrical wires should be secured so that if a person bumps the wire when cleaning or getting something out from under the sink, that the wire doesn’t get pulled loose from it connection screws. The problem with a loose wire is that it is still plugged into the wall outlet and now that the wire is pulled loose from the connection screws, the end may shock someone. Having a proper connector will help prevent this from happening.