It’s only optimal for Anemo supports and for characters that scale off DEF or HP. Anemo supports gain a lot from leveling to 90 – namely, better utility and increased swirl damage. Similarly, level 90 is more valuable to characters that scale off DEF/HP than for those that scale off ATK. This is because there are much fewer sources of DEF/HP stats. Character level is among its only sources, so these characters gain more from leveling to 90. As for weapons, level 90 is always optimal for weapons used by DPS characters. This is simply because a weapon’s Base ATK significantly contributes to a character’s damage.
Level 90 Weapons
Leveling your DPS unit’s weapon to 90 is very important. This is because of its Base ATK. It can’t be gained anywhere else apart from the weapon and character level. Every ATK stat and buff gained by a character will scale based on their Base ATK. So it’s very valuable for maximizing damage. Leveling a weapon to 90 is always a good damage buff – especially for DPS characters.
Anemo Supports
Anemo supports benefit the most from level 90. This is because their lifting ability scales with their level. This refers to being able to launch or lift opponents into the air. Lifting opponents can immobilize them for a short period. Anemo characters can only lift opponents within 20 levels from their own level. For instance, if a level 50 Sucrose faces a level 80 hilichurl, her skill and burst won’t be able to lift the hilichurl – because it’s more than 20 levels higher than Sucrose. So, it’s important to level Anemo supports as your world level increases. Additionally, swirl damage scales off character level. Anemo supports deal damage entirely from swirl reactions. Leveling them from 80 to 90 is a massive damage boost to their swirls.
DEF/HP-scaling Characters
There are fewer sources of DEF/HP buffs and stats – character levels are one of them. Level 90 lets them maximize the DEF/HP stats they gain from levels. To compare, ATK-scaling characters gain very little from level 90. This is because ATK stats and buffs are much more saturated. Basically, level 90 has a lot more value for DEF/HP-scaling characters.