My guess (No informed ideas here!) that they're a legacy feature from trying to support bash scripters and their backticks as well as it's just a quick simple and above all, lazy way to get the output of a command.

my $data = `cmd`;
[download]

Is vastly preferable to

open FH,'cmd |';
my $data;
$data.=$_ while(<FH>);
[download]

(or something of that nature).