In my opinion, language is often more or less relevant to politics and history. Hong Kong is a special region with special history and special political status, which "requires" people in Hong Kong to understand and apply multiple languages. But this actually forms the unique culture of Hong Kong and also results in the important role it plays nowadays.

Here is an article:

https://www.bbc.com/news/world-asia-china-40406429

I used to listen to my grandma talking with her friends on phone in Cantonese. I remember myself being really confused but can still understand some of their conversations. It was like decoding. It's probably because there are still some similarities between Mandarin and Cantonese. Also, maybe I just got used to the Cantonese accent of my grandma which makes Cantonese sounds somewhat "familiar" to me.

Agree...

Adopting Mandarin in various regions is really a time-consuming process. Some people nowadays often argue that it was the prevalence of Mandarin that led to the loss of other dialects. But why is that important for the society back then to adopt Mandarin in various regions?

My guess is that the society back then needed people to understand a common language to keep the country united. After all, language is the information carrier, politically, economically, culturally and emotionally.