What are some uses for an O2/N2 mixture that is more N2-rich than air?
Some laboratories do use, and possibly some processes use "synthetic air", which contains 5% oxygen. This is to carry out oxidation reactions or processes but reduces the amount of excess oxygen to prevent unwanted combustion e.g. if flammable solvents are also being used. Many university research labs will use this if carrying out pressurised reactions.