Mostly for value, that might seem odd at first as hardcovers tend to be more expensive. But I started focusing on buying Marvels oversized hardcovers as they were priced at the same cost as two trades, had the same issues as two tpbs but came in an oversized hardcover. Omnibus editions also price out at similiar to the cost of collecting multiple tpbs.
I also discovered I prefer oversized pages to regular pages for stories I really enjoyed, especially Absolute Editions for books like Watchmen, DKR, Planetary etc. I find I spend more time on the art, and appreciate it more.