interfaces and IComparable

Question

Continuing in my develpment of C# and .Net I am struggling to understand the purpose of interfaces but particularly the IComparable interface. If the interface is implemented it provides a comparison method but what advantage does this provide over the comparison methods String.CompareTo or Int32.CompareTo. These can be called without implementing IComparable. Can someone explain? Have searched internet and not found a useful explanation.
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