Thicker/stronger spokes is one of the keys to make a wheel stiff. A typically 20-spoke wheel built with 2.0 mm spokes is about 8-10% stiffer than a similar wheel built with only 1.8 mm in thickness.
With same gauge width throughout the entire length of spoke. It is typically being used where weight-saving is not the main concern.
The part connected to the hub are slightly thicker in the neck for extra structural strength and stiffness, with a trade-off being heavier comparing to a double-butted or a plain-gauge spoke. It is commonly being used in disc-brake wheels.
These are lighter weighted spokes that the middle sections are thinner. The diameter of these spokes goes from 2 mm to 1.8 mm to 2 mm. The structure is good for road cycling wheel build.
Aero bladed spokes
The middle section of these spokes is flattened for the purpose of reducing wind resistance. It is ideal for time-trialing and high performance wheel build.