“The obligation of Wudu depends on the obligation of prayer.” What does this mean here?

Quality of the Commanded Act: Intrinsic and Dependent Goodness

• The legal demand is linked to prayer, although Wudu is performed before it
• The person must finish prayer and only then perform its Wudu
• Wudu and prayer are the same act with the same components
• The relationship applies only when visible dirt is present