Why Phonetic transcription is important

Every time I want to state my point, I try to resort to those who know much more than myself.
In this case, I want to highlight the importance of NOT using ordinary spelling to represent foreign language sounds.
I'm absolutely persuaded of the fact that we need to teach foreign language pronunciation using procedures that hinder mother tongue interference. Our language (Spanish) is considered a phonetic language since the orthography represents the way we speak (we could discuss this a bit, since we produce certain allophonic variants of phonemes which are not represented by orthography). The use of ordinary spelling could, thus, activate our mother tongue schemata when trying to learn the foreign language (English). The consequence would be that we used Spanish sounds instead of English ones. When we teach English pronunciation we need to help learners to become aware of differences and to produce as native-like  sounds as possible. Think of our vowel system having five different phonemes as compared to the English system having twelve different phonemes. Leave alone the allophones!
Something similar happens with the consonant systems.
Why should we enhance interference by our ill-treatment of sounds?

Here I transcribe part of John Wells' presentation, which you can read complete at: https://www.phon.ucl.ac.uk/home/wells/whytranscription.htm

I hope it will help you to reinforce concepts of appropriate and advisable ways of teaching pronunciation.

“The principal reason for using phonetic transcription is easily stated. When we transcribe a word or an utterance, we give a direct specification of its pronunciation. If ordinary spelling reliably indicated actual pronunciation, phonetic transcription might be unnecessary; but often it does not.

“This is obvious when we consider a language such as English, whose spelling is blatantly irregular; or a language such as Chinese, with a non-alphabetic orthography, whose written form generally does not give any direct information about pronunciation (and of course this applies also to Chinese characters used in writing Japanese or Korean). But even in languages with so-called phonetic orthography, such as Swahili, Finnish or Korean han’gŭl, there may be sporadic mismatches between the sound and the spelling of words, while there are almost always phonetic characteristics of continuous speech that are not reflected in the orthography.

“For the language learner, a passive acquaintance with phonetic transcription enables him or her to extract precise and explicit information on pronunciation from a dictionary, bilingual or monolingual.

“Without this information, a learner risks being misled either by an inadequately trained ear or by the dazzling effect of the ordinary spelling.”