What was the purpose of doing a rough sketch?

To come up with an idea of what you want,and in case you want to change it during the process.

The purpose of doing a rough draft of any art, or project is to make the mistakes early. A rough draft usually does not count for points but it could depending on the project. It is like a pre-test sheet that allows you to realize or draw the mistakes at first hand and then finalize/fix them when you do the final draft.

I hope this helps!