Why IPA beats ordinary spelling
English letters can represent many different sounds — think of the 'ough' in through, though and tough. IPA solves this by giving every sound its own dedicated symbol, so a transcription always maps to exactly one pronunciation. That precision is why dictionaries, language courses and linguists rely on it.
With an IPA transcription in hand, you can pronounce a word correctly even if you have never heard it, and you can write clear pronunciation guides that any reader familiar with IPA will understand the same way.