Magician's Circle can be used to special summon a Dark Magician Girl from your deck, but you need an attacking Spellcaster-type monster, and this means that you can only use Dark Burning Attack on your next turn.
Legion can be used to tribute summon Dark Magician Girl in a turn, which can be used as a surprise, and using Dark Magic Attack can potentially clear your opponent's monsters, allowing you to direct attack right after.
"Dear" Konami, would you mind giving us 1-2 more copies of this card?
This card could, arguably, be better than Dark Magic Attack (Dark Magician version) since Dark Magician Girl is easier to get on the field (requires 1 tribute, can be special summoned with Magician's Circle which, by itself, has easy activation condition).
And even though this card might seem better because it destroys all cards instead of just monsters and that it's a quick-play spell, it's activation condition is way more demanding since, if you read carefully, it says you need to be control MONSTERS (plural) who names ARE "Dark Magician" and "Dark Magician Girl", meaning - you need to have both of them on the field to use it.
Because this card's effect states a "Dark Magician Girl" monster, and not only "Dark Magician Girl", this card can also be used with "Toon Dark Magician Girl" and "Dark Magician Girl the Dragon Knight".