There are many purposes to back purging for stainless steel.

Purpose #1) Protection against rust to prevent corrosion.

Purpose #2) Gas permeation–nitrogen helps remove moisture, which can cause corrosion when in contact with the metal surface. This explains why back purge is good for food containers or tanks that have water constantly being in contact with steel surfaces.

Purpose #3) Paint adhesion- back purging improves the paint coverage, so there’s less chance of bubbles beneath the paint layer. Back purge gas provides better adhesion and blocks out ambient air pressure/moisture that could potentially affect automatic sprinkler activation or dry fire systems (i.e., backdraft).

Purpose #4) Conformal cooling.

Purpose #5) Vapor-phase protection: This is achieved by establishing a uniform, persistent and positive gas atmosphere on the metal surface to protect the steel from corrosion in the event of a fire. Back purging also acts as an effective barrier against harmful gases such as carbon monoxide and carbon dioxide that could form during a fire or be released through out-gassing of furnishings and fixtures. When these gases come into contact with hot steel they can create strong acids, which will cause pitting and rusting on unprotected surfaces. Carbon dioxide also protects stainless steel from the formation of nitrite salts due to reaction with fireproofing agents used for protective coatings (e.g., intumescent, cementitious, and carbonaceous).

Purpose #6) This is achieved by establishing a uniform, persistent and positive gas atmosphere on the metal surface to protect the steel from corrosion in the event of a fire. Back purging also acts as an effective barrier against harmful gases such as carbon monoxide and carbon dioxide that could form during a fire or be released through out-gassing of furnishings and fixtures. When these gases come into contact with hot steel they can create strong acids, which will cause pitting and rusting on unprotected surfaces. Carbon dioxide also protects stainless steel from the formation of nitrite salts due to reaction with fireproofing agents used for protective coatings (e.g., intumescent, cementitious, and carbonaceous).

Purpose #7) Chemical containment–back purge prevents pressure build-up when chemicals are in contact with one another.

Purpose #8) Fire protection – Back purging is used to keep steel surfaces clean, free from fire-engendered contaminants, rust, or corrosion. Contaminants can be introduced into the gas atmosphere of a protected space by various means including outgassing from furnishings, partitions, and other secondary materials made of combustible elements; permeation through concrete floors, walls or roofs in direct contact with the protected area; backdrafts that cause air currents in protected spaces to reverse direction in relation to room pressure so contaminants are drawn out ; infiltration through porous barriers such as floor slabs.

To Know more click here: https://en.wikipedia.org/wiki/Weld_purging