How does “had” add anything? how does one “had better.” how is that grammar, how is it semantically useful. Its just an extra verb someone decided sounded good in middle english that weve been lugging around all this time. Its also not the correct tense for that sentence; for the future perfect tense in which the sentence was written, shouldnt it be “you’ve better?” or perhaps “you will have better?” even that isn’t grammar though, and it doesn’t actually semantically mean “you would be better to believe…” which is what both “you better” and “you’d better” are intended to be understood as. In my opinion.
How does “had” add anything? how does one “had better.” how is that grammar, how is it semantically useful. Its just an extra verb someone decided sounded good in middle english that weve been lugging around all this time. Its also not the correct tense for that sentence; for the future perfect tense in which the sentence was written, shouldnt it be “you’ve better?” or perhaps “you will have better?” even that isn’t grammar though, and it doesn’t actually semantically mean “you would be better to believe…” which is what both “you better” and “you’d better” are intended to be understood as. In my opinion.
tl;dr:
It’s more of a collocation, with the implication being “you’d better believe it (or else)”. But it’s not obligatory, I agree. More of a variant.