Then when children were born & grew up in this pidgin, it became a natural language, a creole. This hypothesis states that vast, polyethnic communities (i.e. empires) often see a national language become simpler as a lot of 2nd-language learners become part of the day-today reality of the language. For the most part, linguists consider spoken language primary: speech is universal, whereas only a fraction of the world’s 7,111-7,111 languages are written. Hence the linguist’s common-sense definition: two people share a language if they can have a conversation without too much trouble.

Examples of such creolized languages are a who’s who of past empires, including Farsi (Persian), Chinese Mandarin, Arabic, & English.[2] So according to these two hypotheses, Mandarin grammar & pronunciation became simpler over time as speakers from different, most likely more complex languages, tried to communicate with each other through a 2nd language, the imperial language. This could explain why Mandarin is so different from other Chinese languages.

Speakers of different Chinese dialects often cannot understand each other verbally, but can still communicate in common writing.  But this written form is not a universal “Chinese”: it is based on Mandarin. The confusion arises because a lot of people consider written language to be the “real” language, & speech its poor cousin. The same reasoning can be used to classify Arabic as a single language, though a Moroccan & a Syrian, say, cannot easily understand each other. Ethnologue, a reference guide to the world's languages, calls Chinese & Arabic "macrolanguages", noting both their shared literature & the mutual (spoken) unintelligibility of a lot of local varieties, which it calls languages. 

To help improve verbal communication possibilities across the country, the Chinese government — over the past few centuries — has pushed for Mandarin (the primary dialect spoken in Beijing) to be taught in schools & used in all public media.  Modern Chinese speakers now often learn both their regional dialect & Mandarin to maximize their communication potential.

Thus, tone is very important in any Chinese dialect. There are a lot of cases when words spelled in pinyin (the standardized alphabetical transliteration of Chinese characters) are the same, but the way it is pronounced changes the meaning. For example, in Mandarin, 妈 (mā) means mother, 马 (mǎ) means horse, & 骂 (mà) means to scold. (Chinese languages are tonal, which means that inflection is used not only to convey emotion as in English, but actually to change the meaning or grammar of a sentence. Cantonese is somewhat more difficult to learn, as it has from 7 to 9 tones, each of which signify different things. Mandarin only has 5 tones. In addition, because of its greater prevalence, it is easier to find Mandarin materials than Cantonese materials to study with.)

Thus, if your goal is to be widely understood, they should learn Mandarin because Mandarin can be understood even in Hong Kong, Macau & Canton (the main regions who still speak Cantonese), Chinese flashcards are a better way to learn Mandarin (Mandarin vs. Cantonese)and more & more Cantonese speakers are learning Mandarin nowadays.  If they really want to be able to connect with people from Hong Kong, Macau, & Canton, they can still consider learning Cantonese.  But they should still know that Cantonese is often seen as more difficult.  Its use of “tones” can be even more challenging to western speakers than Mandarin. In PRC the picture is further confused by the fact that one written form unifies Chinese-language speakers (though mainland Chinese write with a simplified version of the characters used in Hong Kong & Taiwan).