What is the historical significance of the International Phonetic Alphabet?

In the 19th and early 20th century, there were several phonetic alphabets and spelling reform proposals in circulation; Romic alphabet was one instance. Linguists working on different languages had their own transliteration conventions in place, for use not only in citing non-Roman languages, but also for dialectal transcription.

The International Phonetic Association was initially founded to promote Romic; but in 1888 it devised a single, language-neutral phonetic alphabet, the International Phonetic Alphabet, to serve as an international standard. So its historical significance was in overriding the disparity of spelling reform proposals, language-specific transcriptions, and multiple phonetic alphabets, with one international standard, which has gained ground continuously since. Think of it as the metric system for phonetics.

And just like the metric system, there is one country that holds out against it:

Americanist phonetic notation

On the one hand, as the Wikipedia article points out, that’s slightly unfair: the Americanist notation is a systematisation of the various pre-IPA, diacritics-based transliterations, and it is still used plenty in reputable contexts: it’s fine to transliterate Russian <ч> as <č> if you’re doing transliteration instead of phonetics. Like citing a name or something.

Historical linguistics also holds out against the IPA, and uses traditional transliterations instead. So a Sanskrit retroflex n is going to be written as <ṇ>, and not /ɳ/. The Gothic Hwair is going to be transliterated as <ƕ>, and not <ʍ>.

If on the other hand you are using the Americanist notation in a discussion of synchronic phonetics—well, as far as I’m concerned you should be bastinado’d.

But I think that of the imperial system as well.