lang attribute and not widely spoken languagues

I am trying to recommend on the use of the 'lang'
attribute in relation to a not widely spoken language.
(Maori, the native language of New Zealand)
I am struggling to see what benefit using lang="mi"
will have.

These are the reasons put forward by WAI

-Assisting search engines 
-Assisting speech synthesizers 
-Helping a user agent select glyph variants for high
quality typography 
-Helping a user agent choose a set of quotation marks 
-Helping a user agent make decisions about
hyphenation, ligatures, and spacing 
-Assisting spell checkers and grammar checkers 

Can anyone help me out with a real world example of
any of the above OR give a reason apart from any of
the above as to why to use the lang attribute.

TIA
Graham Oliver

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'Making on-line information accessible'
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