Experience has shown that, for most types of electrical equipment, there will likely be a time when the cover will be off, or the door will be open, while it is energized. This applies to CB panels, distribution boards, M-V switchgear, dry-type transformers, disconnect switches, terminal boxes, junction boxes, VFD's, UPS's, PFC's, ........
This is especially the case when a problem of some sort has been discovered, after the equipment has gone into operation. Covers start coming off of lots of equipment, trying to isolate the problem. Covers are also often removed, and doors opened, on energized equipment to perform infrared testing - which is often included in start-up specs and preventive maintenance procedures.
Since a designer, engineer, or installer usually has little or no control over an installation after a project is completed, it is safer to design and install with the understanding that the equipment will LIKELY undergo "examination, adjustment, servicing, or maintenance while energized", at some point in its life. Thus, adequate working space about the equipment should be provided in accordance with the NEC.