The use of Slug field are :
When you give a Post, Page, Category, or Tag a title, WordPress will generate a slug automatically.
The slug is the portion of the URL that refers to that specific page. Generally speaking, slugs tend to be very similar to the original title.
A slug will be different when the title contains certain characters, like spaces or currency symbols, as well as anything else that isn’t a letter or a number.
In those cases, the special character is either replaced with it’s URL friendly equivalent, or left out of the slug entirely.
The most common substitution is in titles that have more than one word, separated by spaces. In that case, each space is replaced by a hyphen ( – ).