The . is the current directory, while .. signifies the parent directory. It makes things quicker at the command line as well so you don't need to type out full paths.
example:
go up 2 directories:
cd ..\..\
or on a UNIX based system, to run executable binaries in the current directory:
./program
A lot of UNIX scripts will also utilize . to represent the current directory, in order to scan for files for example (Perl):
#!/usr/bin/perl
opendir ( DIR, "." ) || die "Error opening current directory\n";
while( ($f = readdir(DIR))){
print("$f\n");
}
closedir(DIR);
It is much more portable if you wish to move the script around to different directories or systems since a directory name is not hard-coded.