Research is a process...
Students need to be aware that when a paper is submitted for publication, a lack of knowledge of the literature leaves them open to professional embarrassment that occurs when some knowledgeable referee cites data in the past that supports (pleasant surprise), or undermines (awful surprise), or duplicates (unpleasant surprise) what you've reported, and says "You really should have known about this work."
When you know "it's been done," before, you know the kinds of techniques that have been used, and the parameters. With mastery of the literature, you can move rapidly in your research because you know how to mine the existing data and find out the important information without having to waste time in the lab discovering how not to run an experiment or rediscovering artifacts, which every technique has hidden from the novice.
Excerpts from how to skate on the edge of the paradigm...and keep from falling off an interview with Nicholas J. Turro, The Spectrum, spring 2004.