Hi @harryjet ! I think it's very usefull for a web developper. In fact, I often have my IDE on one screen and the rendering on the other. It prevents me alt-tabbing every time and allows me to focus in general.
Now to truly answer your question, no it is not "truly" usefull. I saw many developpers work quick and fine on one monitor. I'll say it's more a matter of comfort over productivity.
Hope it helped :).