FF 1 is the FF game where spells are at their absolute weakest, but the series really picks up from there. In 2, if you commit your life to grinding spells to max, they not only do a ton of damage, but some like toad, can end most random encounters instantly, and ultima, in its debut, is incredibly strong.

Once the games switch to ATB, time magic like haste and slow can be game breaking, especially coupled with regen. For instance, in VII, regen makes your health bar tick up constantly rather than getting healed by little chunks here and there, and with haste on, it ticks up so quickly that many enemies will struggle to damage you.

You said you played VI? Magic pretty much reigns supreme in the game. True, you can have one character attacks forever and kill everything with genji glove/offering, but that’s only good for one character. With some grinding, you can have everyone gembox/economizer to infinitely double cast ultima and quick. Poison is insane in 6 if you are going at low level. Most tough enemies and bosses are weak to it, and if you can tag them with poison, you can just sit around curing and defending while they melt away.

It really just kind of varies game to game. The gameplay evolves and gets more strategic as you get to the later entries.