What is the exact use of .Htaccess file? How could it will be helpful from SEO perspective?

[Fabrizio]

The original purpose of .htaccess (reflected in its name) was to allow per-directory access control, by for example requiring a password to access the content. Nowadays however, the .htaccess files can override many other configuration settings including content type and character set, CGI handlers, etc.

Common usages are:

- Authorization and authentication settings;
- Rewriting URLs;
- Blocking by IP address or domain;
- Enabling SSI;
- Changing the page that is shown when a server-side error occurs (404, 301 etc.)
- Cache control.

For SEO perspective it is important for Rewriting/redirecting URLs and blocking some parts of your website from search engines to prevent duplicated content warnings and penalty.