The standard system used to write a language is called its orthography (from the Greek stems: ortho- 'correct' and graphy 'writing'). Even for languages whose writing systems are based on alphabets, the standard "correct" spellings often have little to do with how the words are pronounced.
Phonetic alphabets are designed (and necessary) for writing down an utterance in a way that records how it actually sounded. Ideally, someone who never heard the original utterance should be able to recreate it simply by reading the written transcription out loud.
Fiction writers will often try to give the impression that a speaker is using a different accent by deliberately misspelling some randomly chosen words.
Pets thim animals may be, an' domestic they be, but pigs I'm blame sure they do be, an' me rules says plain as the nose on yer face, 'Pigs Franklin to Westcote, thirty cints each.' An' Misther Morehouse, by me arithmetical knowledge two time thurty comes to sixty cints.
Ellis Parker Butler, "Pigs is pigs"
'Pears lak she should pay some 'tention to her fifth husban', or leastwise her fo'th, but she don'. I don' understan' wimmin. Seem lak ev'body settin' fire to somethin' ev'time I turn my back. Wonder any buildin's standin' in the whole gahdam United States.
James Thurber, "Bateman comes home"
There are several problems with trying to use ordinary English spelling conventions to suggest how a word is pronounced.
First, doing so usually has offensive connotations. Writers seldom use misspelling for the speech of characters they are trying to get you to respect. While the misspellings may help suggest that a character speaks "differently" (from whom?), it usually also implies that the character is stupid or illiterate. (This is especially obvious with misspellings like "sez". So the character pronounces says like "sez", but so does everyone else, so what does the misspelling actually tell us?)
More importantly, English spelling conventions are not consistent enough to be used in a systematic phonetic transcription. Some problems:
The writer of a phonetic transcription facing a particular sound would have to choose between a number of different possible symbols. The reader of a phonetic transcription facing a given symbol could never be sure of what sound it was intended to represent.
There would be problems even if there were some consistency in how a symbol was used. The transcriber might say "the combination 'ay' always means the sound in the word 'day'" -- but would this be the word "day" as pronounced by a western Canadian, by an Australian, by a Londoner? Even if these problems could be solved, English spelling conventions would (for understandable reasons) only be useful in writing the sounds which occur in English -- they would be no help in writing sounds of other languages (or used by language-disordered children) which are not found in standard English.