It works anytime you've got the opponent scared or hesitant to disrespect you (which you should be aiming to establish, because Ragna does not thrive in the ways of air-tight pressure and fast mix-up). There's also the fear of timing their punish wrong and just giving you a free fatal.
Most people, knowing 2C is your fatal, will attempt to hit you after you've already recovered from the whiff (no risk of being fatal'd for them), at which point, any fast poke, 5A, 2A, or 5B, will beat them. That's why 2C whiff is legit.
When in truth, you're -8 off 5C > 2C (w), and -10 off 5B > 2C (w). So of course, it's far from being the greatest thing ever. But like anything, it works when you use it correctly, and mix it up with all your other options.
Funny enough, in both cases, they are actually stuck in block-stun during 2C's active frames, assuming it was canceled into ASAP, so there is no reason for them to fear being fataled.
5B gives 16 frames of block stun on normal, 5C gives 18. 2C starts up in 12 frames, has 3 active frames, meaning they can get hit on frames 13, 14 and 15. Which is silly, since they're stuck blocking during those.
Of course, if you delay it, then they'll actually run the risk of being out of blockstun and running into a fatal, since they'll anticipate that whiff (but not the delay). Instant blocking 5B (13 frames of block stun) or 5C (now 15 frames of block stun) does the same thing, it puts them at more of a risk. Huh. The more you know.