Choosing for your commute
A dry bag gives selected belongings their own water barrier inside an ordinary rucksack or pannier. That is useful when you want a dry change of clothes at work without replacing all your luggage. It also helps organise the bag: clean clothing can stay together while tools, lunch and damp outer layers occupy other compartments.
Buy for the items you need to protect and leave sufficient space to roll the top correctly. A packed bag that barely closes has less useful capacity than its nominal volume suggests. Waterproof fabric does not automatically make a finished roll-top bag suitable for submersion. These selections are for protecting packed belongings from wet conditions, and the manufacturers set limits that should guide how they are used.