Why should I add co-authors on Google Scholar?
In general, I find the Google Scholar profile pages to be elegant and useful. 

Some people choose not to add co-authors to their profile pages. You might not see any benefit. You might not want to play favourites (i.e., who is enough of a co-author to be listed; what if I forget to add someone, would they be mildly offended).

In general, random people will google you and land on your google scholar profile page. Seeing who your main co-authors are might be useful to such people to give them an understanding of where your work is situated and who you are working with. They can then also more easily explore the academic work of your co-authors.