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Concentration
Your whole goal in performing (most) titrations is to determine the concentration of a chemical in a sample. To do so, you make use of a "titrant," a solution whose concentration you already know. If the concentration of the titrant isn't what you think it is, then your results will be meaningless. Consequently, it's very important to make sure the concentration of the titrant in the burette is exactly what you expect it to be.
Impurities
There are two reasons why you rinse your burette. The first has to do with possible impurities. If you share equipment with someone else, such as a lab partner, and she didn't clean the burette as thoroughly as you would, it's possible you could introduce some contaminants into your titrant if you don't rinse the burette first. Depending on the nature of these contaminants, they might have an effect on the concentration of your titrant and the reaction that takes place in your sample.
Water
The second and more important reason why you rinse your burette has to do with water. When you're cleaning your glassware, you use water to rinse it off. If the burette is not completely dry by the time you use it, the remaining traces of water on the inside will make your titrant more dilute and thereby change its concentration. Consequently, if you don't rinse your burette with titrant and there really is some water remaining inside, the titrant you dispense will be more dilute than it should be.
Considerations
If there's one place where haste makes waste, it's in the lab. Rinsing a burette will only take you a couple minutes, but ending up with bad or inaccurate data could force you to repeat a whole experiment --- potentially costing hours of your time. If you're in a lab class, a bad result might translate into a poorer grade. That's why rinsing your burette is just a simple precaution you can take to help ensure accuracy.