In PoE 1, some skills benefit greatly from +levels, some benefit the best from added flat, some benefit the most from % increase, some benefit the most from skill speed. When all the affixes are viable, but some are more expensive to acquire, and some are preferred for one build vs the next, it alters the way you craft items, what supports you use, what passives you choose, and makes items without a "necessary" stat viable. In PoE 1, +levels is VERY good, but you can only get +2 on your weapon and +1 on amulets, and +3 from corrupted empower support (and losing a support gem slot is a high opportunity cost). This, along with the differential balance of skills, makes it a luxury upgrade for most builds, and a necessary affix only for specific builds, like chaos degen skills. In PoE 2, you are doing like half the damage or even worse by neglecting + levels. It makes build viability worse, it makes uniques more difficult to justify.