In the days of hand drafting, architectural lettering was taught to all Architecture and Industrial Design majors. (My graduating class was one of the last ID classes to draft by hand, and they were already phasing it out.)
This type of lettering evolved from the days of drafting pools, where multiple people might contribute to a single drawing, and the lettering and especially the numbering needed to be consistent regardless of whose hand had produced it.
Like many design practices, this was implemented for a simple reason: Money. If you and the guy or gal next to you in the drafting pool write your 7's differently, and the manufacturer or builder working off of your drawing reads one of your 7's as a 2, the resulting error will probably be expensive.
CAD (and poor imitations like the Tekton font) made architectural lettering by hand largely obsolete, but if you missed the boat and want to gain an old-school skill, it's easy enough to learn with a little practice. In this video, architect Eric Reinholdt of 30x40 Design Workshop shows you the basics.
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Reinholdt's even prepared some practice worksheets you can download, for free, here.
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