This was written in response to someone who wondered how much the common Chinese writing system helped people communicate across the various Chinese languages (usually called "dialects").
Until about 1915 almost all Chinese writing was done in Classical Chinese, using conventions utterly divorced from those of any of the eight to twelve living Sinitic languages. The nearest Western analogy would be the 18th century, when most learned works were still written in Latin, but read -- by translation -- in the local vernacular. Subject matter aside, any written text of 1900 would be perfectly intelligible to a literate Tang Dynasty person of more than a thousand years before. Contrariwise, it would be literally impossible to read any written document (except for a few marginal cases) word for word in any modern language whatever and produce anything but nonsense.
As one of the many knock-on effects from the collapse of the Qing Dynasty in 1911, a widespread tradition of writing arose using the lexical and grammatical conventions of modern Peking Mandarin. This made life much easier for the large majority who spoke Mandarin, and who had most of the political and economic power as well. The new baihua ('plain writing') style, however, was almost as artificial for Southerners speaking non-Mandarin languages as Classical Chinese had been. Rather than having to learn Middle Chinese in order to become literate, they now had to learn modern Mandarin. After a period of confusion in language matters, the PRC government nailed down the Mandarin language and the Peking pronunciation standard as official in 1956. It was optimistically renamed putonghua 'the common language'.
By the 1980s, knowledge of Mandarin had become widespread in the South in all public matters. It is the language of schooling past the first year or two, and it is now possible for non-locals to get along with only the standard language, which was certainly never true before. Learning the standard language, however, is not thought of as "language instruction"; what is learned, explicitly, is reading and writing: speaking and understanding are treated as a by-product of this. Similarly, non-locals who must learn a non-Mandarin language think of it more like adapting to local speech habits rather than learning a truly foreign language like English.
It's interesting to note that the native alphabet of Chinese, Zhuyin Fuhao (or informally Bopomofo) was first used to show pronunciations in the official post-dynastic dictionary of Mandarin, the 1919 edition. The spellings attempted to preserve as many distinctions as possible, not only in the dialects of Mandarin, but across the non-Mandarin Sinitic languages as well. The result was a sort of pseudo-Chinese that resembled nothing ever heard before, and that no one except the great Chinese phonetician Yuen Ren Chao was ever able to pronounce. He made a set of records demonstrating the new official pronunciation, but they were hopeless for pedagogical purposes. In the end, the 1932 revision abandoned the attempt, and recorded the actual pronunciations of the Peking dialect.