What is the purpose of the NCPD Hazmat Suit?

No this suit does not serve a purpose other than character aesthetic. Special outfits are uncommon outfits that cover your typical clothes and (as far as I've seen) don't offer additional benefits, armor, or bonuses. While wearing special outfits, you retain the armor and benefits of the other clothing you have equipped, but the visual aspect is overridden by whatever special clothing you have on.

If you have high-quality but really garish clothing, you can hide it with a special outfit. Apart from that it doesn't do much in the game.