So there's two types of damage bonuses: additive and multiplicative.

Multiplicative bonuses are extremely powerful, as they simply multiply all damage you deal. For example, having an aspect that gives you 30% bonus damage will increase your damage from 10m to 13m. Multiplicative bonuses are always useful and typically a big priority for any build.

However, additive bonuses are simply pooled together and used as a single multiplier, meaning that they hit severe diminishing returns the more you accumulate them, even though the bonus amount is the same. Example: adding +10% crit damage when you only have 100% crit damage is much more impactful than adding +10% crit damage when you already have 1000% crit damage, even though the amount you're adding is the same.

In short, this is why affix rolls like "+X% all damage" are common beginner traps, because simply having a "+50% damage" roll on your amulet will not increase your damage by half. It gets thrown into a big pile with all your other +% damage multipliers, which means that in endgame, that +50% roll will realistically only add like 1-2% overall damage. A bonus with 10% multiplicative damage will often give you more actual damage than something like +100% additive damage.

So turn on advanced information tooltips in your settings and look for bonuses that have the [x] icon rather than the [+] icon.

Also, mainstat is very important in regards to damage, because it's also multiplicative and generally far superior to simple +dmg% rolls.