Why use L293D if you don't need to reverse the motor?
Is there any good reason to use L293D if I don't need to reverse the motor but only control motor speed?

No, there is not.  An N-FET low side switch and a catch diode is a much better solution.
Really there's no good reason to use an L293 even if you do need to control the motor direction; FET-based bridges are far better choices, especially in low-voltage projects where the losses of Darlington devices like the L293, L298, etc can represent a fair fraction of the power supply.  But once upon a time someone used an L293 in a project, and someone else in need of a solution copied them, and someone else copied them, and to this day many still unwittingly follow in this mistaken tradition.