Maybe it was just an assumption that I never thought about, but I had always thought STEM to mean the intersection of science, technology, engineering and mathematics. There's already a large overlap between them, so the intersection was also large. So STEAM would not be the addition of all Arts but the intersection of science, technology, engineering, mathematics, and the arts. For example, something like the Mona Lisa is not a part of STEAM, but the aesthetic design of a UI would be. In that sense, the arts matter very much. If we ignore the arts, we have ugly and non-intuitive designs, which lead to lower usability.