When to use snap-off blade knife and when to use trapezoid blade knife?
Trapezoid razor blades are stronger, so they can be used for cutting thick or very tough materials better than snapping blades. Examples where I use this blade:


Cutting carpet or vinyl
Cutting/carving wood where cleanness of the cut isn’t important 


Snapping blades are super convenient when making lots of cuts on materials that dull or gum up blades quick and the material isn’t really thick or tough.


Cutting packing tape
Cutting thin/medium gauge cardboard