When William Blake originally published his works, his images were inseparable from his words. Nowadays, we happily sever the two completely, reading his words as if they were a complete entity, and casually glancing at the images whenever there's enough time or interest.
Naturally, such behavior is certainly understandable, as his words contain so much depth on their own, even without knowing the images exist. More practically, Blake's handwriting doesn't make for the easiest reading.
Thus, this site serves as a compromise: providing easy viewing of the images, with the text superimposed on top. This way, the images are never just a passing fancy, they're an integral part of each and every word you read.
Additionally, while suffering through complicated texts by yourself can be a reward of its own, it sometimes helps to work through Blake's work in conjunction with others. As such, this site also has the feature to add footnotes to any point in the text, which can be viewed alongside the transcriptions.