Don't know why there are Total Freight Orders

Every factories making their freight and sent it to commercial buildings.

But it is more effective when there is trade depots. Factories can send more freights and faster.

You can check the freight orders(commercial buildings order freights)at your city status bar.

But I don't understand why commercial buildings order freights.

It still fine if there are no industrial buildings.

Of course, industrial buildings create many jobs but nothing to do with commercial buildings.

Leak of Total Freight Orders demands industrial zones, and it is annoying.

Is there any reason for this?