1 Dialect, as a branch of Middle Chinese, has a brotherly relationship with Mandarin.
The diversity of dialects reflects the diversity of cultural customs in China.
Many local operas are based on dialects. Without dialects, they would lose these precious arts.
Studying dialects can study the changes of ancient Chinese and the relationship between ancient phonology and modern phonology.
We can't suppress dialects just to promote Putonghua. There is no conflict of interest between them, only nepotism.