The chicken feed I buy is still in paper bags, but the cat food I buy is the new plastic weave kind.
I saved them, and was soon glad I did -- we had an unexpected, unusually heavy rain hit (8 inches in one hour). The weatherman was able to warn us ahead of time of the heavy rains and flash flood warnings.
Since the henhouse is on a lower elevation (only place we had to build it), that kind of heavy rain threatened to flood the henhouse.
So we got together all the plastic cat food bags we could find (including some donated by a neighbor) and filled them with sand -- instant sandbag. We sandbagged the entrance to the henhouse, and the other areas most likely to admit water into the structure.
And guess what? Those cat food bag/sandbags kept the floodwaters out of the henhouse!
I eagerly await the point when my favorite chicken feed changes to the plastic bags -- I'll be able to save up enough "sandbags" to build a small levee when that happens!