The 19″ rack as a long story and like most “normalized” artifacts, some of its dimensions are inherited by past uses/components (telephony?), some of which are not used anymore.
The 19″ rack is globally used as a standardized server enclosure (or computer cabinet) in data centers. Build in steel and usually heavy, it is nonetheless designed to be mobile (mounted on wheels and easily packable on pallet and/or within containers for transportation). This massive use leads to the common esthetic of the data center, with straight lines of cabinets containing multiple servers. A 19″ rack usually do the same thing at a very small size than a full data center: it physically protects servers, organizes their dispatching in a very rational way and control air flows & climate/temperature.