We assume this connection is offered by the airline; the question therefore seems odd as no airline would want to offer connections that do not normally work, given the financial responsibilities.

In this case, the good thing about connecting via YUL is that any bag checked at YYZ will not have to be retrieved for US customs during your connection. So, you get the convenience and speed of domestic check-in/security at YYZ and pre-clearance during your layover: Often, this alone is worth not flying YYZ-USA nonstop!