Simple historical question: Why was the acme thread angle designed with a 29 degree pitch instead of a round 30 degrees, as is done with metric trapezoidal?
Clearance, Cleophus!
Both male and female threads have included angles of 29 degrees, and this slight gap allows some clearance for smooth mating and lubrication if required. In addition, when wear takes place at the narrow end of the thread, the larger surface areas of the sides mate to each other better.
That explains why one would prefer trapezoidal threads (in general) over square threads.
Can you explain why the Acme trapezoidal thread standard uses 29 degrees, but the metric trapezoidal thread standard uses 30 degrees?