It was a key add-on for marketing purposes, as the LC575 was used extensively in schools and as the machine needed to do double duty there as a presentation device (the next big thing around 1993). If you look at the vintage advertising materials contemporary with the machine over on vectronicsappleworld.com, the feature was heavily publicized.

Now, it was also possible to pop something like a Lapis LCPDS video card into one of these and get monitor spanning capabilities with up to a 20" screen connected — and with some of those handful cards, accelerated video at that — but that is not what we've been talking about.