Primary reason is Oxidation. Without plating or a lacquer coating, all brass will eventually turn green'ish. Lacquer isn't an ideal solution for a commercial product as the client would have to keep up with the maintenance themselves. Instead, a coat of nickel gives the metal a semi-permanent corrosion protection (nickel tarnishes as well, but that's something else altogether). Without any protection applied, fingerprints can also become a permanent fixture on the metal unless you wipe them off right away – finger oils accelerate oxidation. Sandblasting accelerates it even more, with the metal oxidising pretty much instantly when exposed to air.
The other main reason is strength as brass is a very soft and malleable alloy: nickel has an HRC of around 50 to 60 which protects the metal from dings and scratches. Keeps your product looking good for longer, as you can also polish it from time to time.
There's also the benefit of nickel plating's slight levelling effect: it can mask small imperfections and scratches in the machined parts if a thick enough coat is applied (≈ 20µ), to give the impression of a perfectly polished part underneath.
micaddict said:
Why do brass microphones almost always get plated anyway?