The disagreements ITT at least answer the question I came away with after scanning this post—"if these are almost all pronounced the same, why the different diacriticals?"

The partial answer being, some dialects retain differences and they are significant. My own accent is not terrible especially for an American raised when and where I was, but I internalized it early enough (just through middle school instruction, sadly) that I don't even know if I pronounce them all the same... I'd have to read some passages and inspect.

But I was hoping for a little more by way of explicit discussion of the why, which I infer is largely: diacriticals are mostly artifacts of etymology which at some point became ossified and absent a Dudens-like change in prescriptive heart, are here to stay, mostly unvoiced indicators of language evolution (like the silent k and gh in English knight).