Period parts usually cost more than most restoration budgets can bear, unless you salvage. Affordable restoration is largely what salvage makes possible: reclaimed fixtures, hardware, and glass are how a person without a large budget gets real old-house material, real age, and real use, rather than the thin imitation that's the only new alternative at the same money.
The real thing, at a reachable price
The problem it solves is cost. Genuine period fixtures, hardware, and glass, bought new as high-end reproductions, run expensive, and true antiques through specialty dealers run higher. For a homeowner restoring a modest old house on a modest budget, both can be out of reach, which pushes people toward cheap modern parts that only gesture at the period. Salvage offers a third way: real old material, at a price the mass-produced imitation and the boutique antique both undercut from opposite directions. A salvaged fixture is genuine period material with real age, and it often costs less than a good reproduction of the same thing, because it's a used object rather than a new luxury one. So the budget restorer can have the old part instead of the imitation, which reverses the usual rule that age costs more. And it means old-house parts aren't reserved for people who can afford boutique restoration; they're within reach of ordinary owners of ordinary old houses.
Salvager's Note: Price a restored salvaged part against both a good reproduction and a boutique antique of the same thing. Salvage often beats both, giving you the real, aged article for less than a new copy, which is the opposite of how authenticity usually prices.