I don't know if this is the right way to explain it, but the way I understand shields is that when you successfully shield, the opponent behaves as if they hit something, so they go into their hit-stun animation, while you do not (or can cancel out of it) and have some invuln frames to input a move that will counter. EX-Shields have more invuln, I think.
C-Moon does whatever on EX-Shields, and special or super cancels on regular shield. H-Moon auto counters, or manual counters/special/super cancels vs projectiles, and F-Moon manual counters or special/super cancels. This stuff usually leads to a counter hit, or an untechable launch, both of which are comboable at prorated damage, unlike shield bunkers.
If you know they are doing a frame trap or staggering predictably, you can use shield, among other things, during the gaps. Also, shields are easy to time vs meaties on wake-up, and IAD resets. However, they are also easily baited. Some moves need to be shielded multiple times before countering.
In a lot of cases, it's difficult to reversal during pressure b/c you have to input directional motions. But, shielding is just hitting a button and doing a follow up counter. So a good answer to some char's pressure, like F-Warc, is to shield it.
I'm just trying to think of more places where shields are commonly useful...
I've seen people use shields in awkward situations where players aren't sure who has advantage, or as an 'oh shit, I just fucked that up/miss-spaced that!' button to 'avoid' miss-timed punishes.
If you put out some move that covers a certain space, they reacted and spaced around it, and you are now at disadvantage where hitting an attack button will lose, but you know they will hit an attack button to counter you, you can shield it and counter.
-TexasTim-