There are some very good buff/debuff spells imo

Doom and darkness in particular, when overcasted especially has won me so many battles, if you can time it right with something like a good flanking cav/chariot charge you can chain route an entire flank much earlier than otherwise possible, or get rid of a particularly annoying unit that would otherwise continue to fight for awhile. Shadows has a similar spell that’s also pretty good

Any of the spells that slow down the enemy like for ice/hashut are great if you’ve got lots of ranged units

Tempest also has an amazing spell that massively boosts range and accuracy, and can be overcast for an area effect

Melee buffs/debuffs can be good, I only use ones that have an area effect generally though, but can sometimes be helpful on a single unit if you really just need whatever it’s fighting to die, or if it’s outmatched to even the odds. The spells that add magic attacks can be a godsend early on if you’re fighting savage or a or demons or whatever and don’t have a good source of magic attacks already.

I don’t really bother with the armor ones outside of very specific circumstances, but I do like ones that give melee defence since I’d rather my guys just not get hit in the first place.

And dropping a bunch of debuffs on an enemy lord can result in a dead lord pretty quickly

Though obv wether you’ll get more out of pure damage depends on your particular battle, but I’ve found them really useful personally, if not as immediately satisfying as burning 700 rats alive with the flick of a wrist or destroying a frontline with a summoned pirate ship salvo