Easiest way to see this difference is watch the news in Cantonese -- it is mostly 书面语 (standard) mandarin but pronounced as Cantonese, whereas day-to-day spoken Cantonese is completely different in structure and vocabulary. What do you think gets written in HK movie scripts? Definitely not standard written mandarin.
Regional languages in China have separate writing system when you need to write what is actually spoken.
https://en.wikipedia.org/wiki/Written_Cantonese#Vocabulary
https://en.wikipedia.org/wiki/Fuzhou_dialect#Writing_system
Easiest way to see this difference is watch the news in Cantonese -- it is mostly 书面语 (standard) mandarin but pronounced as Cantonese, whereas day-to-day spoken Cantonese is completely different in structure and vocabulary. What do you think gets written in HK movie scripts? Definitely not standard written mandarin.