The vCenter Server Events sub-system is an incredibly rich and powerful interface that enables customers to monitor, alert and even trigger additional actions based on a particular event. One such example that I have written about before is to key off of a VM provisioned event and automatically apply security hardening settings when the VM is created or cloned. This can be useful if customers are not taking advantage of VM Templates or if a VI Admins manually creates a VM from scratch, you can still ensure you have a compliant VM deployment through the use of Automation. You can either poll for the VM created event and then execute a script as shown in this example or you can automatically trigger a remote action by generating an SNMP trap when the event actually occurs.
The possibilities are truly endless on what you can do with vCenter Events and for the complete list of all Event types, you can refer to the vSphere API documentation here. One thing to be aware of is that not every operation within vCenter Server generates an Event, one example of this is when a Folder object is created or deleted. You can use vCenter Server Tasks sub-system to query for this info but there is not a respective vCenter Event that you can key off of to generate an Alarm for example. This was something I had noticed myself and assumed it was a limitation of the platform or feature teams that publish VC Events.
Recently, this question came up again from a customer who was looking for a way to trigger an alarm every time a VM Folder was created. I took another look at this and came to learn about a more generic type of Event that can be used to create an Alarm for such use cases where a native VC Event may not exists called a Task Event.