There are hands where a tile is used in a pung/kong as well as a single tile. For those cases, when calling a discard, you may want to hold one natural tile back to use as your single (since you can't use a joker for it) and use a joker for your pung/kong.
A good example is 2026 line 2: if one of your pungs of dragons is soaps, you may want to hold back a soap to use as your zero in "2026".