Quick breads in foil pans
There's no reason you have to put foil pans on a tray, they can be put directly on the rack if you like. There's no reason not to either, it's a matter of convenience. If you want a scientific answer a tray will interfere with air circulation a little, but not enough it makes a real difference to baking times or the end result.  

Using a tray can be a good idea if the pans are too flexible to handle easily, and also if you are baking many of them at once putting many on one tray makes it faster to get them in and out.