Well, if you're building a wall yourself, you can't use a whole truck load of concrete all at once. You pretty much have to mix it incrementally.

I am trying to understand your post. Is it cheaper per yard to hire a concrete truck than to mix concrete (with free aggregate/sand) yourself?

Presumably, to get the truck pricing you have to buy by the truckload which is too much concrete for some projects. If doing it onsite was cheaper at quantity, you'd never see a concrete truck on the road -- all the big construction sites would be saving money by mixing onsite.