A typical air hose will have a checked coupling on one end, and an air connector on the other. If these terms confuse you a bit, please visit their pages on www.about-air-compressors.com.
If you are using high demand air tools and have plumbed them with an air hose or air hoses, after you have done your work, water vapor in the compressed air in those lines could condense out. The next time you connect that air line to an air tool, along with the first burst of compressed air, could come a slug of water. Not good for the air tool if it gets to it, for sure.
In one of their compressor manuals, Campbell Hausfeld suggests, and I thank them for the suggestion, that when the work is done for the day, if you can hang the air hose connector down (the connector is open all the time) if there is any water in the air line, it will drain out while hanging.
Not a crisis if you do not do this, but certainly a potential plus if you do.