packet forwarding query

hello,
Following is code snippet from ip_forward function from ip_forward.c
in kernel source code
.........
dev2 = rt->u.dst.dev;
mtu = rt->u.dst.pmtu;

/*
* We now generate an ICMP HOST REDIRECT giving the route
* we calculated.
*/
if (rt->rt_flags&RTCF_DOREDIRECT && !opt->srr)
ip_rt_send_redirect(skb);

/* We are about to mangle packet. Copy it! */
if (skb_cow(skb, dev2->hard_header_len))
goto drop;
iph = skb->nh.iph;
...............

when router takes packet to forward does above code is written to
check if
next dst hop has different network device or if it is using different
hard_header_len so that skb_cow is called?
whats use of skb_cow in ip_forward?
I am using 2.4 series kernel.
regards,
rahul