Mydakota:
It definitely can run two steppers or 4 DC Motors maybe this link will explain more
I know that. And I can just about carry a 25kg bag of coal - it's much the same problem.
Controlling steppers with a h-bridge is hard work for an Arduino because it has to calculate the timing and order of the pulses for each stepper coil.
A specialized stepper driver takes that load off the Arduino just like a wheelbarrow makes it easier to move coal.
Also, and importantly, reading an encoder puts a high computational burden on the Arduino so you need to relieve it of other avoidable jobs.
Unfortunately the marketing guys like to claim that their super-duper-shield can do everything.
...R