That's pretty much it. There are some units, or items for characters, that make them immune to non-magical attacks, or give other special rules against non-magical attacks. Against those targets, having magical attacks comes in very handy.I have a friend who recently got the Wood Elves and Vampire Counts books, and they refer to a couple of rules I'm not familliar with and can't find in the 7E book. Certain units of Wood Elves count as having 'magic attacks': what does this do? Certain VC units are insubstantial/ethereal or something like that: I can't find teh rules for that either. Am I just blind, or are they hidden somewhere?
Things like units with the "Ethereal" rule, or items, for instance there is a Bretonnian item that means you can only take one wound in any particular phase, but it doesn't work against magical attacks.
It doesn't come up that often, the list of things where magical attacks makes a difference just isn't that long. But it is still very handy when you have to go up against something with a rule like that. Mostly, magical attacks only come from normal magical weapons which characters purchase, but sometimes having a whole unit that can dish them out is useful. That's where rules like "attacks count as magical" come in handy.
Hope that helps somewhat