what is the usage of IPrimaryKeyConvertor

[li...@wachovia.com]

Some tapestry user said, IprimaryKeyConvertor is for:
----
An interface defining how the items iterated upon by this component will 
be stored in the form as Hidden values. This interface allows only the 
primary key of the items to be stored, rather than the whole item. 
---

I think getPrimaryKey(objValue) is enough to achieve that purpose, why 
need getValue(Object objPrimaryKey) ?
I think getValue() should enable me to get the object that is correspond 
to a row in a table directly without having to go through reconstructing 
the object using primary key, but I can't find such sample or I completely 
misunderstood its usage, please help!