The most widespread modern use of the seventy-two is to identify a personal guardian angel from a date of birth. It rests on the zodiacal division and it is more recent than most of its users suppose.
The mechanism follows from the arithmetic. Seventy-two angels across three hundred and sixty degrees give five degrees each, and five degrees of the ecliptic corresponds to roughly five days of the solar year. So each angel governs a short window of dates, and a birthday falls in one of them. Refinements assign a second and third angel from the hour and from a further division, producing a personal triad.
The zodiacal placement itself is early modern. Lenain's tabulation of 1823 already gives each angel a range, and it is the direct ancestor of the modern date tables. The elaboration into a personal-guardian system, with sub-angels of the hour and detailed characterological readings, belongs to twentieth-century French occult publishing and to the English-language market that followed it.
None of this is present in the medieval material. Rashi describes a way of extracting names from Exodus. The Bahir and the Zohar develop the seventy-two-fold name theologically. Neither has anything to say about birth dates, and the notion of a guardian angel assigned by nativity is not what either is discussing.
Two things are therefore true at once, and both should be stated. The birth-date system is a genuine and coherent development of an older tradition, worked out carefully by people who knew the sources. And it is roughly two hundred years old at most, in its detailed modern form considerably less, and it should not be described as ancient.