Leading on from jdporter & chas' answers:

1) because you may want to pass a scalar, a list and a hash to a subroutine. You can do that with:

testfunc($scalar,\@array,\%hash);

sub testfunc
{
 my ($scalar,$arrayref,$hashref) = @_;
}
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2) Because hash references are fundamental to OO perl.

3) (Elaborating chas a little) Because a data structure like:

my $appointments = {
                    monday=> { "10:30"=>"dentist",
                               "11:30"=>"doctor" },
                    tuesday => { "10:30" => " planning meeting",
                                 "16:30" => " funding deadline" }
                    };
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can be very useful.

4) Because once you start using array refs and hash refs, you'lll wonder how you ever managed without them.

I may be missing something, but I rarely have occasion to use a scalar ref in the simple $ref = \$var; form.

Update:Struck out 'hash' in 'hash references' at blazars suggestion - TIMTOWTDI.