Hey @Laureen Youngblood, I can see your point.
The material question is not meant to waste the contractor's time or pull a fast one on them. It's to avoid using a contractor that is bidding their labor low to get the project but making it up on the material cost. Yes, they do get contractor pricing but not all contractors pass that saving on to the customer. If the contractor is itemizing his bid, he'll already be taking measurements to provide you price.
Example: The GC charges $2/sqft for 1,200 sqft of flooring for a total of $2,400.00. As the homeowner, I now know that I need at least 1,200 sqft of flooring material. I can now get that myself without him going to the store with me. I haven't wasted his time nor made his job any more difficult.
This point is really for finish materials (tile, flooring, cabinets, etc). I wouldn't necessarily recommend doing this for the drywall, framing material, insulation, etc unless you have the knowledge of these items.
I've done plenty of jobs where we only did the labor and those jobs tend to run smoother because we don't start until they already have their material paid for.
At the end of the day it depends on how much work the homeowner wants to do and the relationship they have with the contractor.