It's just a convenience. If you know your daily buffer requirement and your daily evaporation rate, you can make up a container that will serve your needs for a week: multiply one by the other for the amt of buffer.
I use Kent KH Buffer in my topoff. My autotopoff delivers it to the sump.
This doesn't relieve you of testing, but it means one less thing you have to add, if you're having to add it daily or even several times a week.