I think the idea is that users who roll more dice, or are inclined to have "more" entropy rather than less, are better off with 24. Yes both 12 and 24 reduce to 128 bit but 24 provides a stronger checksum and better opportunity to capture randomness from physical entropy. Just IMO

As far as I know, the maximum effective entropy of a Bitcoin private key reduces to 128 bits, regardless of how many bits you use for creating the master secret: https://foundation.xyz/blog/make-12-words-the-standard

Kruw

Oh really? That's news to me. I have heard people say it is unnecessary but not that it doesn't provide extra security. Why is that? That sounds like an over-generalization.