By Definition.
or it wouldn't be Necessary.
Wise, Strongly Suggested, Vigorously Encouraged, perhaps, even Greatly Recommended.
But if its not a Mother... its not Necessary.
I should note they also make grey nylon/plastic ones. They are cheaper. DO NOT use them. A screw with good purchase will pull right thru one - meaning a good wind on all the surface area you are about to make with metal roofing will concentrate a lot of force where the screw head meets the...
Check your pole diameters. Then get these (sized appropriately, of course) to wrap around the underside of your poles and screw/bolt to wood above. That will allow you to attach purlins above your metal framing you can then secure your metal roof to.
Or accept minor leaks, pre-drill holes, and...