Why necessarily " " ?
 

I was looking for the lines that begin with MANPATH (not MANPATH_MAP!) in the file man.conf.When I entered:
, I didn't get any output.But when I tried:
it worked. What was the problem with my first snippet code?
Thanks!

The shell will interpret special characters unless you escape them (with \) or enclose them in quotes.

For example, the command:
Will print the contents of the current directory, because the shell will evaluate the * and replace it with each file in the current directory (seperated by white space, of course).

While the command:
Simply prints *

You should look at the bash manpage. There is a section about this called "quoting".

Yes, I know the quoting problem, but this is what I don't understand:
Why doesn't happen the same when I enter the commands from my first post?

If you try:
The output is:
The ^ character has no special meaning.

In your first post, you were using \>

The > is a special character (used with pipes). The shell will see the \ and assume that you meant to escape the > character, not that you're using grep regular expression syntax.

See for yourself using the arguments for grep from your first post. Try:
(the output is ^<MANPATH>)

(the output is ^\<MANPATH\>)

What can I say...A big "Thanks"!

you\'re\ welcome
:)