Linked from
The 9 pages that link to Atomic mass unit, each with the reason it gives.
Atomic weightRelated: Atomic weights are commonly reported in this unit.
Mass defectRelated: Nuclear and atomic mass defects are commonly reported in this unit.
Electron massRelated: Electron mass is commonly expressed as a small fraction of this unit.
Whole-number ruleRelated: The rule states atomic masses in multiples of this reference unit.