Convenience.
If I need to put something like 1.7 qts. in the Kawasaki on my JD, I don't need to be messing around with a 5qt jug, a 6qt box, or a 55 gal drum.
If I need to carry some in the tool box of my truck for top off, I don't need the 5, 6, or the 55.
This view carries across the board...