They are the most faithful representations of the original art (or at least in case of the IDW AE-s but I think the Artifact and Gallery editions are going for the same things as well).
They scann the original pages in color and super high def so you will get every pencil line, coffee mark, everything. Then they print it on the same or very similar paper that the artist used and in the same size, which could result in hilariously gigantic books like the MAD AE.
You can look up some on youtube. Pricey stuff but for collectors and art lovers it's worth it I think. If I could afford 100-120 pounds/book I would buy the Rocketeer and Born again ones and the upcoming LoEG AE as well.