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You can see the difference by looking at the traits:
ray of frost has an attack trait (and has you make a spell attack roll)
disrupt undead does not have an attack trait (and the enemy is making a save, you're not making an attack roll)
As a side effect of this, a one-action ranged attack can be nice for spellcasters, since you might still have an action left after spellcasting and no MAP yet. You're probably not going to do amazing damage, but it's a nice freebie.