I've just had to turn my telly on for the first time in several months to answer your question! (I subscribe to Now TV but I rarely use it).
A dedicated Now TV device (or, these days, a compatible device, such as a Roku TV Stick, with Now TV loaded onto it) turns a 'non-smart' TV into a 'smart' one. So it gives viewers access to BBC iPlayer, ITVX, 4 on Demand, My 5, UKTV Play and Pluto TV.
Therefore a Now TV device (even without any form of subscription) can be of considerable benefit to someone who only owns a traditional ('non-smart') TV set but offers nothing to someone who's already got a 'smart' TV (as they'll have the above catch-up services available to them anyway).