Since spells are also randomly assigned to weapons, knowing which ones to keep will help you in deciding whether it is worth keeping a more powerful weapon with a bad spell or a slightly weaker weapon with a great spell. Having a combination of bad spells and weapons can affect your survivability.
Normally, spells that stop enemies such as Freeze, Slow, or Stun are essential to slow weapons like the hammer. Buffing spells like Rage are quite useful for quick weapons like the one-handed swords and gauntlets since it allows you to dish out more damage in a short span of time. For other weapons, wide-area spells like Flame and Whirlwind will increase your destructive capacity, especially when upgraded to LV3.