String_Access in unbounded string handling?

[Blady <p.p11@orange.fr>]

Hello,

String_Access is defined in A.4.5 Unbounded-Length String Handling:
7          type String_Access is access all String;

and note:
75  The type String_Access provides a (nonprivate) access type for 
explicit processing of unbounded-length strings.

I wonder what String_Access is for and what could be "explicit processing"?

Thanks, Pascal.