The idea is that the client code will search for the link element whose rel attribute is equal to the known value the client is looking for, and then get the uri attribute for the request. In other words, the client knows the name of the uri for the functionality it wants, but it doesn't have to know the uri. This allows the uris to change if needed.

It's like using constants instead of literals in your code. If the literal value has to change you only have to change the constant definition, not every place in the code where the value is used.