Please offer some advice.

Based on my observations, this skill only affects the damage dealt by combat spells, but not summoning spells.
However, Nature doesn't have any combat spells. Does that mean it's an unnecessary skill?

Only Chaos magic has another additional skill that solely affects the effectiveness of combat spells. Otherwise, in all other schools, efficiency skills affect spells that are defined numerically. In Nature, this is the number of summoned creatures. You can read more about it here /topic/8363/