In a recent article in the Daily Maverick, Tim Cohen wrote about the possible effects of a downgrade of the South African (SA) Government's USD-denominated debt by Moody's to sub-investment grade at the end of March, 2019. This downgrade would move SA government debt into "junk" status as the other two global rating agencies have already downgraded it to this level.

Clearly explain what do rating agencies such as Moody's do. Why is it important for the South African government to be rated by them?