Focus on one task with git
All the reasons of this article are important, but it is not the most important one for me.
I is so easy to digress from your goal and end up having 3 ou 4 different problems being solve at the same time, specially when you are a beginner, the code starts to smell, to get messy and your own organization falls apart and you no longer can keep track of what is done and what is not.
Git (os any other SCM) helps you solve this because you have to actively commit the changes you made (Dropbox automatically syncs, so you have no control) adding a description to what have you done. This helps you focus in one task at a time.
If you are not doing one thing at a time, you should.