Any question you ask people in a Web form requires them to parse it, formulate a response, and then input their answer in the affordance you have provided on the form. Being vigilant about every question you ask allows you to remove questions that are not absolutely necessary. And the fewer questions you ask, the better the odds are of people completing your forms quickly and easily.
A recent case study published by Imaginary Landscape highlights one example of how much better things can get. The company performed a comparison of an 11-field Contact Us form with a 4-field Contact Us form. They found contact form conversions increased 120% when the number of fields was reduced from 11 to 4 (a 64% decrease). Furthermore, the fields removed had no impact on the quality of the conversions. You can can access the full case study at: Fewer fields in a contact form sharply increases conversions (PDF).
As this example highlights, it pays to carefully examine all the questions being asked in your Web form for opportunities to eliminate unnecessary inputs. Do you really need to ask this question? Is it information that you can get automatically? Is there a better time or place to get an answer from people? Though this process appears tedious, you may be surprised when you discover what you can leave off your forms.