Motivation for -rpath-link

[Vladimir Prus <ghost at cs dot msu dot su>]

Hello,
I'm somewhat confused about one sentence in -rpath-link documentaion. It says:

   When using ELF or SunOS, one shared library may require another.
    [....]
    When the linker encounters such a dependency when doing a non-shared, 
    non-relocatable link, it will automatically try to locate the required
    shared library and include it in the link, if it is not included  
    explicitly.

Why this behaviour is desired? For example, I have exe "main" which links to 
shared library "b", which links to shared library "a". Why it is necessary to 
include "a" in link to "main". It is just in case "main" uses symbols from 
"a", or there's some deeper motivation?

Thanks in advance,
Volodya