What is the role of an intrusion detection system (IDS) in a security posture?

Answer 1

To detect and alert on potential security breaches or unauthorized activities.

Answer 2

Having IDS & IPS in your environment will always benefit. IDS helps in detecting the intrusion an attacker or group is trying to intrude into the organization, such Intrusions attempts are also useful in terms of blacklisting the IP's and understanding geolocation. It detects suspicious activities and gives alert to the admin.