Because that function is trivial to write and it isn't the only or even the obviously right choice.

sub first_or_all {  wantarray ? @_ : $_[0] }
sub last_or_all {   wantarray ? @_ : $_[-1] }
sub list_or_aref {  wantarray ? @_ : \@_ }
sub aref_or_list {  wantarray ? @{$_[0]} : $_[0] }
sub pairs_or_href { wantarray ? @_ : {@_} }
sub href_or_pairs { wantarray ? %{$_[0]} : $_[0] }
sub all_or_concat { wantarray ? @_ : join $,, @_ }
sub all_or_die {
    return @_
        if  wantarray;
    return $_[0]
        if  @_ <= 1;
    require Carp;
    local $Carp::level = 2;
    my $sub = ( caller(1) )[3];
    Carp::croak(
        "Don't call $sub() like that in a scalar context" );
}
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(to list just a few off the top of my head that I've heard people argue are wise choices)