Because the maps work the same either at the computer or on paper. The computer-based Mind Map gives you more features and the ability to dynamically alter the map, but the basic functionality remains the same. This is very frustrating to me. The Mind Map transcends this limitation and lets you focus on your data rather than the technique. Each time you look something up on your Mind Map all of the data is reinforced, increasing your retention.
Electronic organizers carry the promise of making our data portable.
Besides there is just something about seeing things in my own handwriting that makes it personal. So I use handwritten Mind Maps for the most part and use the MindManager software when I need to share things with others or quickly convert my map to a Web page or outline for further development. Creating maps between different disciplines and exploring the relationships I find has been very useful to me. I am a Webmaster. Combining thoughts between the two yielded many parallels that weren't apparent before the exercise.