They were made in the UK in the 50s and 60s as serving tanks to hold bulk carbonated beer in the pub cellar, a use that didn't last when consumers demanded a return to "real" cask ale. In the early days of US brewpubs in the 80s, they were bought at scrap and repurposed for just about every imaginable use in the brewery, as were old dairy tanks and anything else a resourceful brewer could adapt, in the absence at the time of proper brewing equipment at that scale.