In my experience, yes and no. It helps and, depending on the cake, you can tell a difference in texture. I have some recipes that just don't work without it. I've done a few with All-Purp and they work out fine. I think of it like substituting pastry flour or bread flour.
You can Google for some suggestions specific for the type of cake. Sometimes adding something (like wheat gluten) helps in some recipes
You can make your own if you don't have any on hand.
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