Business lead generation. Give the reader a taste of the content and have them fill out their info to get access to the entire report. Then you build a list that you can market to later. Hubspot does this well - http://www.hubspot.com/marketing-resources.
As a rule of thumb nowadays content should be more open and free to people. Use it to be recognized as an expert in the field and generate business that way. It is increasingly difficult for someone to "steal" or plagiarize work so I'm not sure you need to be so on top of protecting content that you miss bigger opportunities.
Give the user a reason to want to complete the form in the first place. Like others have said don't be afraid to give your visitors a taster of what's to come in the form of a partial book or a screenshot of your Whitepaper. You might also want to think about other techniques that are commonly deployed in online stores to get people to purchase. So think about things such as allowing users to rate or comment on your content so that other visitors can get unbiased opinions before they download. You should also use things like progressive profiling so that you are only ever asking for minimal information at any one time, this reducing some uncertainty people have about giving away their personal details.
Referring to the comment about using bite-size pieces of content to give your prospect a taste, think about how you can repurpose the content into different formats for various platforms. Does your content lend itself to images? Then post images or create images with quotes from the content to post on Pinterest, Facebook, G+ and Instagram... if that's where you ideal audience hangs out. Prefer video, do 2 minute video tips based on the content with a call to action to get the report. Hope those ideas help!
