And this applies to other domains. I do grant writing for nonprofit and public agencies. It's easy for me to have a main document, an RFP, an organizational background file, and a research website (like the Census) open at the same time. We use 27" iMacs and side monitors for good reason.

Even in writing a novel, it's often easy for me to have a character list, a plot outline, and a main document open simultaneously. It's often not "necessary," per se, but it can be helpful.