Two years ago, when I first joined Michael Walker's Data Science & Business Analytics Meetup, the form asked (and still asks) "What important truth do very few people agree with you on?" My answer was "Data should never be deleted". At the time, I had no idea what Data Science was and had barely been introduced to Big Data, but it was a dictum I lived by, much to the consternation of my bosses over the past two decades when it came time to approve purchases of hard drives.
Well, I may have to update my profile, because it seems more and more people are agreeing with me. As I blogged on the January, 2014 Boulder/Denver Big Data Meetup, the discussion format came to a consensus that all ingested data should be kept intact as-is as an immutable data store, and that processed data should be stored in some kind of data warehouse for the actual analytics. I wrote then that it was good to have that pattern, which was in the making for a couple of years, finally codified as a pattern.
It's even more solidified now. The two most common motivations given are: