let vs double-parentheses

[Peng Yu]

Hi,

https://tldp.org/LDP/abs/html/dblparens.html

It is not clear what is the difference between let and
double-parentheses. Can they be used interchangeably? Or in some cases
that one can not replace the other? Of the two, which one is
preferred? Thanks.

-- 
Regards,
Peng