A developer is an oxidant cream product containing hydrogen peroxide. It helps in opening up the hair cuticle, which allows the color to penetrate deep into the hair and lasts for a long time. It is absolutely essential to use a developer while coloring your hair.
Grey hair, in general, is more resistant to color, but if you are doing a deposit only color you could get away with 10 volume developer. However, usually, 20 volume developer works better for grey hair. You can use 30 volume developer for high lift color.