Paying cash for the hard copies of the books offers more than just the guides. You sometimes get a poster or map with it and if you purchase the same guide online through the publisher you are paying the same price for something you have downloaded and don't get any other extras with it. Also if you are playing a game like Skyrim and need to find out something fast you have to open your laptop (Turn it on) or start up your computer and bring up the guide which will take a few minutes.
If you own the hard copy of the guide you can flip through the pages quickly to find what you are looking for. Plus some of the online guides that you download have a chance to be lost if something happens to your computer and than you would have to go and re-download it. Also if you are playing a game on your PC you aren't going to want to minimize it to load up a guide that might freeze your current game and cause you to lose stuff or start over.
So personally for me I prefer the hard copies for the reasons listed above and
1. You can read the if the power goes out
2. You can skim them fast and easy
3. You don't have to worry about losing any information
4. You own a hard copy!