I'm known around these parts as the guy who does mounds of research on seemingly unrelated topics. My favorite list includes the following: Ancient Chinese Weapons, Victorian Social Structure, Weapons of the 18th and 19th Centuries, Steam Engines, Victorian Clothing for Men and Women, Victorian Building Materials, and the logical progression of a thunderstorm. I also instigated a discussion on Twitter about the proper usage of a grappling hook ... but that's not important. The reasoning for why I research these things is simple really, and a powerful truth that any writer should acknowledge.
You Don't Know Everything
Invariably, there will be a part of your novel that the story requires, but you know nothing about. Case in point -- in CALLARION AT NIGHT, there's a scene where Moriah (the MC) is being tortured in a fairly gruesome way. Now, I know very little (read:nothing) about the mechanics of torture, the best feasible kinds of torture for the situation, or really where the best places are to make deep cuts so the subject will bleed without dying.
I do, however, have friends with medical knowledge enough to bounce ideas off of. They'll be able to tell me in enough detail where I'm completely off base or whether I'm spot on. With research, you gain this same ability.
Say your steampunk aeronaut flies a giant balloon across the world. Well that's all fine and dandy, but how does the flying machine actually work? Pedal power? Steam power? Some kind of magical source that amounts to authorial handwaving? And what does the balloon look like?
Research answers these questions about power sources and mechanics and such. If you did a good deal of research on this topic, you'd find that the size of the balloon needs to increase every time you had extra weight. Soon you end up with either a balloon roughly the size of Montana or some sort of handwaving that amounts to praying the reader doesn't notice the implausibility of the machine.
Because for every author who does that, there's a reader like me who will dislike the book because he or she can tell the author didn't do a scrap of research on the topic. And sometimes, research will even bring up a new plot point that you hadn't thought of before. Double whammy, that.