Let's examine a simple, but interesting situation:
We have to defend against this riichi. Theoretically mangan is enough for us to finish on the first place. But making a mangan from this hand is practically impossible, especially when we see an early riichi from a Japanese pro who is now the fourth. Risking our second place for an illusory first is a bad play.
To discard or seems fine, but in fact is a mistake. It's the first turn after riichi and we don't want to pay for ippatsu. The early riichi by the dealer on the fourth place doesn't have to contain ryanmen or another good wait, it may be kanchan with discarded from ryankan: or .
The only choice is tsumo-kiri: .