I read that in theory the iii chord is considered useless. True/Why?
Not useless.
The iii can sound a fair bit like I (because they share two tones) and like V (because they share two tones), so it makes a decent substitute if you want to plug a minor sound in for part of a major or dominant chord. It can lead naturally to vi, because the roots make up a v-i cadence. And for a typical bass line progression, I-III-IV-V is often used: for examples, see "Fun Fun Fun" (Beach Boys), "Crocodile Rock" etc.
Functionally, the iii has no tension, so it doesn't "drive" a chord progression. But neither does any other non-dominant chord - so it's only useless if you're thinking in terms of a dominant-tonic analysis (and in that line of thinking, all the other chords are 'useless' too!).
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Yeah, it came from the kids on Reddit. I usually don't listen to them, but they were ganging up on me. Thanks.
I just thought of something off the top of my head. Is there a chord that you can substitute for VII Dim that would lead to the III. If so, is it common? Examples?
I just thought of something off the top of my head. Is there a chord that you can substitute for VII Dim that would lead to the III. If so, is it common? Examples?
Yes, the secondary dominant - V7/iii. In the key of C that would be B7 - Em. Leonard Cohen does exactly that in Hallelujah "... the baffled king com (B7) posing Halle (Em) lujah."