Question

what are reasons a author may use dialect

Answer 1

Using dialect is a tricky thing. If someone over does it, it can be irritating.
But one use would be if the author was trying to establish a character's voice.
Especially if there were many characters. This could be done by using different speech patterns.

Answer 2

I know they recommend NOT using a lot of slag or incorrect spellings repeatedly.

Another example of when to use it though would be different words or phrases such as referring to a soda as pop, as in the Midwest.

Answer 3

http://www.justaboutwrite.com/A_Archive_Uses-Abuses-Dialect.html

This articles focuses on the don'ts, but mentioned the two uses. The first example shares what I was referring to with the Midwest example, but in this case, it is the basic words for the setting of the book, which is Scotland.

So even though they don't use another language, they use the phrases.

Answer 4

The next example would be including an accent. Obviously you wouldn't want to make something so foreign that the reader couldn't understand what was going on, but again, but including some key phrases and pronunciations, the reader could decipher where the person was from based on the dialect used in the book.