This is what my husband said they were.

He was a career firefighter and said that a lot of engines carry rubber bladder for such purposes.  He was on an engine company once that carried 5-gallon buckets as they got a lot of small beauty bark fires they were about to extinguish with a couple of buckets of water.

The reason the firefighters were using these rather than pulling a hose is because they were low on water and 'charging' or filling the hose takes hundreds of gallons of water that is wasted when they disconnect it.