Basically, your spells scale with a percentage of your spellpower. The higher your spell rank, the more percentage you get.
I believe max rank of frost bolt benefits from 80% of your spellpower. 300 spell power / frost spell power would increase frostbolts damage (max rank) by 240.
It is too simplistic. But +spell damage does increase damage of spells. Direct damage spells benefit more. +Spell dmg does very little for say Arcane Explosion or Blizzard for example.
You can read more about it if you google “wow classic spell coefficients” or such if you’re interested in learning more about it and which spells scales the most with +dmg.