Question about <selector>

Hello!

What is the purpose of <selector> in identity constraints? I understand that
<field> paths are relative to the <selector> path, but what does the
<selector> give me that I couldn't get by specifying a full path for each
<field>. In other words, what is the difference between

<key name="mykey">
   <selector>foo</selector>
   <field>bar/shoes</field>
</key>

and

<key name="mykey">
   <selector>.</selector>
   <field>foo/bar/shoes</field>
</key>

 or

<key name="mykey">
   <selector>foo/bar</selector>
   <field>shoes</field>
</key>

Thanks,
Sean