Why would topolects get described in terms of Mandarin? (声母变异、韵母变异)

Question

[Perhaps this is a "which came first - the chicken or the egg?"-type question - but I'm left quite puzzled why anyone would describe things in this manner.]

Why would anyone (or any material for that matter) describe dialects/topolects in terms of Mandarin? Most topolects/dialects would certainly be much older than MSM (Modern Standard Mandarin) - as a, relatively, new 'invented' language.

e.g.: 广安方言与民俗词典 describes a bunch of 广安方言 in terms of:

声母变异

and

韵母变异

even going as far to describe some words as:

自创音节

here's some examples:

声母变异: “敲” kao1 - “qiao1”的音变。指打，击。

韵母变异: “眉” mi2 - 眉“mei2”的音变。如“眉毛”、“峨眉豆”等。

自创音节: “□” pie4 - "指破烂，坏，不好。常写成“孬”。

I highly doubt that these are "变异" seeing as these pronunciations outdate the modern "standard" ones - so why would anyone go to the trouble of describing it this way? besides the fact that MSM is the, now, "standard"?

Answer 1

Yes or No.

In Chinese linguistic study, there are three rules of distinguishing dialects from languages:

1. Linguistically, mutual intelligibility
2. Historically, derivation
3. Politically, anything except the standard

Dialects are picked out mainly because MSM gets politically recognized as a standard in China Mainland.

In practice, a minor reason is that they do not have matured or accurate writing system.