Let's put it this way:

Without Swiftcast, you have to stand still for 2.5 seconds to cast a standard spell, all the while Weight of the Land is forming under you

With Swiftcast, you can sprint while casting a spell and it will still fire off.

Without Swiftcast, you have to stand still for 6 seconds to cast Raise, all the while Landslide is coming at you

With Swiftcast, you can sprint while casting Raise and it will still fire off

Form your own conclusion