I've noticed recently that the use of 儿 differs depending on the dialect but I'm not too sure why. It is present in 普通话 (Standard Chinese) and some gov docs, as well as taught to L2 learners, but never fully explained. Sometimes it is used to change the meaning of words ( 一点 vs 一点儿）。 I know there's a sociolinguistic factor - at least in 北京话 （Beijing dialect), where overuse suggests lower class. It's much less common in Southern China, but where it does appear it acts completely differently: It can be used as a diminutive suffix and can cause duplication of words (盘 vs 盘盘儿). I've been told that it can also cause tonal sandhi in adverbs. Apparently 儿话 does not appear in Taiwanese, and I do not know whether it exists in other Chinese languages such as Cantonese. I just want to understand 儿话 and why it is more common in the north, is it a loan from Mongolian or something that existed throughout Mandarin and has dropped out of use in recent generations?
The origin of 兒化 as a morphosyntactic suffix is somewhat debated, but the overall picture is well known.
The use of a diminutive suffix is cross-lingustically very common: compare book > booklet in English, café > cafecito in Spanish, конь > конёк > конёчек in Russian. Even Korean has a derivational process that's similar e.g. 괴 > 고양이.
The use of diminutives in Chinese had become fairly widespread in the Song dynasty, although from our knowledge of Chinese poetic rime schemes, it was still a Late Middle Chinese, full-syllable pronunciation, something along the lines of /ȵiɛ/ or /ȵʑjɛ/ (in level tone). This phenomenon is often called 兒尾 érwěi.
We see parallel 兒尾 érwěi developments in diminutives across many non-Mandarin Chinese topolects, where the lexeme for 'son/child' is employed:
Standard Cantonese: 仔 as in 肥仔 fei4 zai2 'fatty'
Taiwanese Hokkien Min Nan: 仔 as in 桌仔 toh-á 'table'
Fuzhounese Min Dong: 囝 as in 蟲囝 tè̤ng-giāng (/tʰøyŋ⁵³⁻³³ ŋiaŋ³³/) 'insect'
Hangzhou Wu*: 兒 as in 瓢羹兒 /biɔ213 kəŋ44 l̩213/ 'spoon'
*Although this classification is disputed, with many seeing the topolect of Hangzhou as a Mandarin variety.
As Chinese moved into the Yuan dynasty, with its use of Mongolian and its -/r/ suffix, 兒 and other syllables like it (而, 耳) underwent a form of metathesis, resulting in syllables ending with -/r/ being prevalent in Old Mandarin. This also affected the phonetic shape of 兒尾 érwěi, evolving into the modern phenomenon of 兒化 érhuà.
Although Beijing Mandarin forms the basis of standard Mandarin, its use of 兒化 is widely parodied in the media. It is part of what distinguishes the two as a bit of a shibboleth. Hence the sociolinguistic complexities, where correct use is a marker of the Beijing identity, but also of non-standard Mandarin.
Taiwan Mandarin officially does use 兒化 érhuà in its standard, although with fewer lexemes than in Mainland Mandarin. However, Taiwan Standard Mandarin is on a continuum with other levels of Taiwanese Mandarin. The Taiwanese Mandarin accent commonly found in Taiwan media and on the streets of Taipei pretty much eschews 兒化 completely, even if the rhotic phoneme in e.g. 然後 ránhòu is retained; any 兒化 is perceived as distinctly Mainland.
Note that in other topolects that are classified under Mandarin, e.g. Chengdu or Shenyang, the lexemes that use 兒化 may differ considerably to the Mainland Standard or to Beijing. However, most of these do use 兒化 as a diminutive, and there is considerable overlap: 一點兒 'a little' is rhotacised in Beijing, Chengdu and Shenyang, although not in Wuhan or Liuzhou.