In all our surveys, we asked the respondents to give us their estimate of actual, or expected improvement in productivity. In 2002, the average expectation of productivity improvement due to the use of multiple monitors was 46%. Productivity expectation in 2012 dropped a bit to 42%, and in our recent 2017 survey it stayed the same at 42% average expected productivity.
Multiple displays give you a resolution multiplier: you simply cannot get the same resolution on one big screen as you can with two or more. Two monitors are often less than the price of one big one and provide more resolution.