Your SQL database can be huge. So many tables, so much data, combining it can be hard. Even once you've sorted out what data you want it can be a real task to visualize what joins you need so that you only grab the data you want. The more efficient the join the better. I found this nifty image on SQL joins that I think does a great job at showing how the various joins work and does a great job explaining how to use them.
I hope this helps someone else as I know I am always having a hard time figuring out which joins to use and when.