I know zero about the file.
Don't be disappointed! I offer questions, you fill the story!

I never create this file in my site, but I do see several of them under different directories.
1. Who cteated these .htaccess files for me for what reasons?
2. If I need to modify .htaccess manually sometimes? why?
3. what does the file used for?
- I guess redictorion uses it, right? any other cases?

from the source directly

.htaccess files (or "distributed configuration files") provide a way to make configuration changes on a per-directory basis. A file, containing one or more configuration directives, is placed in a particular document directory, and the directives apply to that directory, and all subdirectories thereof.

http://httpd.apache.org/docs/2.2/howto/htaccess.html

cpanel creates the htaccess file automatically when account is created I believe, the file has no directives in it when created this way.