Until, and if, you join the thieves' guild, bags of grain are the lowest weight container in the game (100 compared to chests that range from 200 to about 500, etc). Some people carry a container to keep their inventory uncluttered, or for another reason...
Stealing.
If you open a container inside your inventory and drag and drop items into the window it is not considered stealing. It is counted as stealing when you remove the items from the container, though, so make sure there are no NPCs around to see you when you do.
A potential problem doing this is described below;
weight bug and disappearing chests (broken links are fixed in a reply, and the screenshot links re-fixed in another)
