advantage of using aggregate

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UML

I read your article about ddd and it's very usefull for me.I want to ask that what is the advantage of using aggregates (as it presents group of objects with one root)

as explained in your example customer car and tier

suppose my code is

public class customer
{
public Integer GetScrapTiers()
{
//this returns no of scap tiers for a customer who has a car
car.tier objcar=new car.tier();
objcar.getNumberofTier(Cust_id,scrap);//for cust_id tyres of type scrap
}

}
[this is an aggregate we can say]
public class Car
{
private Tier
{
private integer getNumberofTier(id,type)
{
//implement..
}
}
}
}

Now is in this way we will implement aggregate or in some other way please explain.