IGP LDP sync drives the point that a interface should not be used untill it has both LDP and IGP adjacency formed across it.
Some scenarios where this can be helpful would be:
When an IGP adjacency is established, the router begins forwarding packets using the new adjacency before the LDP label exchange completes between the peers on that link.
If an LDP session closes, the router continues to forward traffic using the link associated with the LDP peer rather than an alternate pathway with a fully synchronized LDP session.
There are many documents on Cisco.com and can be found easily searching for LDP IGP sync.
HTH-Cheers,
Swaroop
