hit rating reduces the chance that your spells wil be resisted based on lvl, as lvl 70 12 spell hit rating = 1% to hit with spells, so if you are only grinding you'll need 12x4 = 48 spell hit raiting in order to metigate 4 of the 5 inate resistance a lvl 70 mob has, if you raid the raid targets count as lvl 73 and therfore have 17% resistance, 16 of wich can be metigated, 192 is the spell hit raiting you are aming for.
raiders prioritize spell hit higher than spell damage, because a) they have so high base damage that spell hit increases their dps more than spell damage. Generaly you only need 1k shadow bolt damage for spell hit to pwn spell damage.
secondly when banishing is important then spell hit is your true friend!, because there spell damage dosen't help you
do not confuse spell hit with spell penetration, spell penetration subtrackts from the enemys resistances (fire/shadow for warlock) where the hit raiting subtrackts from "lvl resistances"