Depending how it's used, that kick can have a number of effects. Used early, it stops that foot from planting where it's expected, which takes them a bit off balance (if you're extraordinarily lucky, it takes them a lot off balance), which makes a step-through cross unlikely. Used with some force, even a bit late, it pushes the knee back, which again changes their balance, making the step-through less likely. If someone is familiar with the kick, they can manage to step through it, but most folks won't be.
Of course, miss the kick, and you're standing there for that cross. I tend to use it when folks are entering/closing to punching range (rather than during the punch), because the penalty for it missing or not working is much smaller.