“儿ér” is mainly used in the dialects of Beijing and northen China, so it can be discarded in mandarin. "儿" is used a lot when we want to say something small or unimportant, like "事儿 shìr" （something unimportant ）， “碗儿 wǎnr” （a small bowl) , and "儿ér" is also used to soften the tone and show the fondness for somebody or something, such as "小丫头儿 xiǎo yā tour" (a little girl), "小狗儿 xiǎo gǒur " (puppy). However, if you can use "儿ér" properly, it will add credit to your Mandarin, since which is a standard form based on the Beijing dialect.

in south fo china, people dont say "儿“ . so they wont say 你去哪儿 then, in south of china, there is an influential dialect which is called: 粤语（yue yu） in this dialect, people say: 你去边度 to express "where are you going" it prounences: ni hui bin dou (attention: not ni qu bian du 粤语 is different from the official chinese language) so if people from south china say "where are you going" in the official chinese language, first they should translate 你去边度 into 你去哪 or 你去哪里 then prounence it. so they just say : 你去哪里 or 你去哪 because this is the official chinese language. 你去哪儿 is the north way to speak chinese for them because they never say "儿”.

Actually I come from southern China,when I use Mandarine talk with others,I train myself to add r after a phrase(not all,use it proper time)cause in China,many people test their Mandarine level,and the Mandarine is mostly basic on Beijing dialact,and some of my foreign friends they also tried on it,but it's not a compulsory thing to add or not add,we could understand if you don't,so that's depend on you

the use of 儿 should be divided into 2 streams 1) 儿 is as part of the word: 女儿, 幼儿园 etc 2) 儿 is the northern melody suffix 那儿, 点儿, 会儿 etc

Yup, regional differences exist just like multiple phrasings of the same idea exist. For instance, your 等一会儿 could be expressed with "Please hold a moment," "Gimme a sec," "Hang on," "Just a minute" depending on the context and who's speaking.