Erhua can be used to show that a word is a noun, that something is small and to soften the tone of what is said, ect.
My most common usage of it is in 这儿 (zhèr)，那儿 (nàr) and 哪儿 (nǎr)。(They all mean the same as 这里，那里 and 哪里) Then you have 玩 (wán) -> 玩儿 (wánr: pronounced like war). 小孩子(xiǎoháizi) -> 小孩儿 (xiǎoháir: prounced like xiaohar).
You can here some examples here: https://www.youtube.com/watch?v=VbziFBH8m7w
Yes, that would make a lot more sense from a translation point of view and make the learning process much more Intuitive. "A little bit" has basically the same meaning as in Chinese in this case, it's simply a way to be more polite. The qualify of these courses varies a lot and it's really frustrating and a lot more complicated than it needs to be at times.
I'm not a course contributer, but -- changing the word order in English changes the meaning. "Be slow a bit" would be understood as "be slow for a little while," ("a bit" modifies the duration of the speed) not, "be a little slower" ("a little" modifies the speed). Does that make sense?