I've seen the word "utility" used to describe clothing as early as 1898, but the term gets its last big hurrah during World War II, particularly in Britain, where utility clothing was endorsed by the government. I suspect the whole idea of utility clothing carried such emotionally difficult connotations by the time the war was over that nobody ever wanted to hear the term again, and I don't think anybody has.
Thus, a utility robe is serviceable and economical to make and maintain. Simplicity's three-in-one strategy serves them well with this pattern. Here is the chirpy description from the instruction sheet:
Three distinctly different utility garments for 'round the house wear may be made from this one SIMPLICITY hand cut pattern.
Style #1 With its feminine frill is a dainty, useful morning apron or house-frock.
Style #2 A careful choice of fabrics creates a mannish, tailored, lounging robe.
Style #3 The Hooverette is the ideal garment for the practical side of housekeeping.
This fairly brief passage gives us a lot of information. First, these garments were for household wear only, although I suspect it was acceptable to go out in the back yard to hang some laundry or to nip on down to the end of the driveway to leave a letter in the mailbox. Also, even for garments worn only at home, a desire for femininity is recognized (or perhaps just marketed to.) Lastly, a morning apron and a Hooverette were either perceived to have different functions, or met differing ideas of acceptable or desirable dress. (For example, my grandmother wore house coats, I wear robes, they look pretty much identical to me, but I would probably shop for a robe pattern rather than one for a house coat.)