Professional dart players use tungsten darts because tungsten has the ideal properties to make the highest quality and best performing darts. Tungsten is a very hard, dense metal, which can be used to make darts thinner, more aerodynamic and more durable than cheaper brass darts which create more drag and are less stable in the air.
The barrels of tungsten darts are made from an alloy (a mixture of 2 or more metals) of tungsten and nickel in varying ratios. The higher the tungsten content, the thinner the dart can be made for the same weight. For example, an 80% tungsten 24g dart will be slightly bigger than a 90% tungsten 24g dart. A thinner set of darts is usually considered an advantage because they can be grouped more closely together which makes hitting those 180s a little easier. Professional dart players usually use a 90% tungsten dart, although some may use a higher percentage like 95%.
In most cases, the higher the tungsten content the higher the price, so for players that are fairly new to the game an 80% tungsten dart is usually a more cost effective option to work out their preferred weight of darts before moving up to higher density tungsten darts with a slimmer profile.