This may be for a variety of reasons. The most common reasons are:

• Requests to use images
  • People frequently find this site by Googling for images while writing a presentation of some sort. Inevitably, requests to use these images are made. Inevitably, I agree to the use of these images for non-commercial educational purposes (see the Terms and Conditions for more details). For commercial use, I also inevitably agree - but the paying public would probably prefer good-quality images. Making contact gives me the opportunity to provide a better quality file, or to make adjustments to it in preparation for publication.
• Negative comments or requests for correction of site content
  • Ideally, I would prefer for these to be made in the form of a comment on the specific page you are disagreeing with; that way they (and ensuing changes to content) are easier to keep track of.
• Positive comments
  • Thanks!