Burr and blade grinders there is a difference.
Blade grinders use the inertia from the blades to effectively throw the beans around and cut them.
This leads to inconsistent density and grain size.
Not such a great issue for French press or Cafetières.
However an espresso or any forced pressure maker needs a consistency of grain size to ensure the pressurised water passes evenly through the basket.
If there were inconsistent grain size then the water would take the least resistant path and thus not provide a consistent and controlled extraction.
A burr grinder effectively "mills" the bean similar in concept to a flour mill though with erm blades…….you get the idea.
The net result is a consistent and regular grain size…………tah dah
Erm I sound geeky, I'm off