Personally I find bar bags essential for touring
1 - You need a map for touring, the best place by far to keep the map is in the map case of a bar bag.
2 - All you valuables (camera, money, passport, etc) fit in the bar bag which can then be instantly removed when popping into a shop or whatever, also camera/money etc are to hand when needed
So minimum spec for a bag is:
Small as possible (5L should be OK)
Must have map case
Must have quick release system (such as R&K)
Must be at least semi waterproof (bog standard plakky bag will do if it really tips it down)
Should have as many internal and/or external pockets as possible
Should have a shoulder strap that can be shortened or folded so as not to be in the way
You need to consider lighting - as bar bag will often block your current set up (I have mounted a space bar upside down so my lights are below the bar bag)