What is gained by the extra level of map indirection recommended in help?

The 'write-plugin' section of the help recommends the following
3-level map approach:

map ,c <Plug>TypecorrAdd
noremap <unique> <script> <Plug>TypecorrAdd <SID>Add
noremap <SID>Add :call <SID>Add(expand("<cword>"), 1)<CR>

I understand how it works, but it wasn't immediately clear to me what
was being gained by the middle level of indirection: specifically the
map to <SID>Add. What is the advantage of this strategy over the
following 2-level approach?

map ,c <Plug>TypecorrAdd
noremap <unique> <script> <Plug>TypecorrAdd
\ :call <SID>Add(expand("<cword>"), 1)<CR>

The help says...
"If another script would also map <SID>Add, it would get another
script ID and thus define another mapping."

While this is true, isn't this sort of script-uniqueness already
guaranteed by the use of <SID> in the rhs of the 3rd mapping? I.e.,
:call <SID>Add(...)<CR>

Thanks,
Brett S.

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