It allows them to track you.
Example: site a could generate a 64bit random id to you. Then the site hosts 64 images - 1 for each bit in an ID. The site has you load only the images for bits in your your ID that are 1, and not the images for bits in your ID that are zero.
Then when you go to site b, site b has you load all 64 images, and times how long it takes to load each one. Those will allow them to calculate your ID. Now they know you are the same user from site A. Site B will also repeat what site A did to you, and post that into to site C D E etc etc, so they can all track you, link your account together etc etc.