Why do some feet screw on and others clip-on

Why do some feet screw on and some clip on?

Traditionally feet screwed onto the machine and lower priced machines usually still have screw-on feet. However, over the years a quick and easy snap-on system was developed to make changing feet more user friendly. Manufacturers thought that if changing could be done in a simple way it would encourage sewers to change the feet for each job - and they were right!.
Feet that need to be screwed onto the machine use a large thumb screw, although tightening this screw with a screw driver will ensure that the foot does not become loose with the vibration of the machine during use.