Mages can no longer use Diamond Arrows since November 23, 2020, due to Vocation Adjustments. "As a result of this change, all other vocations can no longer use bows and crossbows to shoot ammunition."
In the past, mages would you Diamond Arrows because it would increase the chance to activate offensive charms. "Diamond arrows damage an area of 21 fields centered on the target (2 fields of radius), just like old Fireball Rune (before update in 2007). Since the target will always be within the damaged area, you will hit it even when you miss it." SOURCE: TibiaFandom It means every turn you have about double the chance to activate your offensive Charms, in the end, it represents a lot of extra damage dealt.