Why is testing your recipe good practice?
Simply put, you don't want to mess your system up! Many times, a recipe can be incredibly complicated, and just one job can be processed through a whole lot of steps. If you click "start" instead of test, you will have a hard time troubleshooting multiple jobs as compared to just troubleshooting one job to make sure the rest will run fine.
Therefore, before you start running tests on your new recipe, please make sure of the following
1) Your recipe has a trigger event that it can pick up. i.e. Make sure that it can find what it needs to find!
2) Make sure that your Since parameter is set correctly.
Once ready, simply click on Test Recipe. It will pick up the first trigger job that it finds and run through it. Once that has been processed, the recipe will stop. You will be able to check out what has been done for this specific record.