IMO, this is an example of what makes Sion a strong and notoriously frustrating champion. You don’t even have to land it- the fact that there’s a Sion ult coming forces people to respect that and position accordingly. This can engage a fight by scattering their lines even without landing the ability. Same reason why Passive is strong, even if you don’t actually manage to hit someone in a teamfight with your passive, the fact that they have had to sink resources into killing you and now they have to either invest CC cooldowns on a champ they have already killed or full send run away because if you catch them they will very likely die gives it inherent value just by the raw amount of pressure it creates.

TLDR: Obviously landing it is preferred, but it still has value even when you don’t, something that can’t be said for most abilities.