Why he matters disproportionately
Marcus Vitruvius Pollio is not the greatest architect of antiquity and may not have been a distinguished one at all. He matters because his book survived and everyone else's did not.
De architectura, in ten books, is the only substantial architectural treatise to come down from the ancient world. It covers town planning, building materials, temples and the orders, public buildings, private houses, water supply, geometry and astronomy as an architect needs them, and machines. Almost everything modern scholarship says about ancient architectural theory begins with it, and that dependence is itself a problem: a single voice has become the whole ancient position.
The man
Little is established. He served in some engineering capacity under Caesar, and he addresses the work to Augustus, whose sister he says supported him. He mentions one building of his own, a basilica at Fano, which does not survive.
He writes as a practitioner reaching the end of a career, anxious that his profession is undervalued, and inclined to explain forms by telling stories about their origins.
What is in the book, and how to read it
The famous elements — firmness, utility and delight as the qualities of good building; the orders and their proportions; the anthropomorphic origin stories for Doric, Ionic and Corinthian; the account of pozzolana and of why lead pipes are objectionable — are all genuinely useful and none should be taken as reportage.
The origin stories in particular are aetiology. The orders predate him by centuries, and no earlier source explains them as he does. His proportional schemes are also more regular than the buildings: measured surveys of surviving temples show variation the canon conceals.
Afterlife
The treatise was known through the Middle Ages and became prescriptive in the Renaissance, when Alberti, Palladio and others took it as the authority on classical building. Through them the Vitruvian vocabulary reached almost every public building in Europe and its colonies. The drawing known as Vitruvian Man is Leonardo's illustration of a passage in Book 3 on human proportion.
That afterlife is why a Greek temple's vocabulary is legible on a nineteenth-century bank, and why this platform treats Vitruvius as the hinge between ancient practice and later European building.