I know some site, which they place a meta robots noindex, no follow in most inner pages. Why did they do that? Is this good for SEO?
there may be some pages which are of no use to user or don't add brand value to your site like Terms of Use. if you don't want these pages either to be indexed or your precious PR passed to them, then you can use noindex and nofollow. but it's better to write an entry in robots.txt file for this purpose
They add no-follow as they don't want their link juice to flow away, and to prevent link hunters from posting irrelevant comments/posts in order to get a link.
if you dont want your links form a certain page followed by SE robots for whatever reason (minimize external links, no follow on affiliate links, etc) then you use nofollow. noindex is used if you dont want SEs to index your pages and some of the reasons could be to avoid dup content penalty, no value of those pages in terms of SEO but useful to visitors, etc
from the point of view of the owner, it is a way to protect his site from backlinks going to competitors ( who leave comment on his articles)
if you do not want other links to get backlinks from your site you can put nofollow tag. if you do not want some pages like terms of use or contact us to be indexed at search engines you can choose noindex tag. nofollow will also save your PR juice and keep it for your site only.