Here is what the literature can do for you:
1. Give you an evidence-based measure of effect to use in an a priori power analysis. It will show more empirical rigor on your part if you use the values from the most current and highest-quality evidence available.
2. Help you choose the "gold standard" outcome that is most generalizable and applicable to your audience and peers. Using the best outcome measure available increases the internal validity of your study as well. If the same outcome is used in many studies, then it has more validity evidence to back it up. This, again, shows stronger empirical reasoning on your part.
3. Allow you to ask a question that is relevant and that will generate new knowledge. You will be able to pass the "So what?" question with ease when you know the literature. You will know what new knowledge needs to be generated and how it is relevant in the context of the existing literature.
4. Help you choose the correct research design to answer your research question. If you find that the literature only has observational evidence related to your area of interest, then you can make the informed decision to employ a more complex design to yield causal effects.