From a security standpoint, intrusion detection systems (IDS) and intrusion prevention systems (IPS) are complementary to firewalls because firewalls are generally access-control devices that are built to block access to an application or host. In this way, a firewall can be used to remove access to a large number of application ports, reducing the threat to the servers..
Increased network availability :Provides network wide, distributed protection from many attacks, exploits, worms, and viruses.
Faster remediation :Pinpoints the source of network attacks faster and takes corrective actions closest to the attack
IDS and IPS sensors look for attacks in network and application traffic that is permitted to go through the firewall. If it detects an attack, the IDS sensor generates an alert to inform the organization about the activity. IPS is similar in that it generates alerts due to malicious activity and, additionally, it can apply an action to block the attack before it reaches the destination.