Venkat,
In a nutshell this is a stand alone server vs a load balanced set of servers, and cluster may not be the proper terminology here. A clustered domain orchestrator is 2 or more Process Automation servers behind a load balancer (F5, NGiNX, Apache are the three we have certified with). If you have a single domain orchestrator and you are beginning to notice a performance decrease and your workload is increasing, you may want to consider a load balanced setup. This is just one of the ways to help with performance, but depending on the server resources may not be the only issue.
Advantages and disadvantages are going to be on a case by case basis. If you don't really require a second Process Automation node, then server management would fall under the disadvantage column.
The obvious advantage is for a fail-over situation. If server A suddenly becomes unavailable, all work will be processed on server B, and there won't be an interruption in service - unless of course your workload is so heavy that one server cannot handle the load.
All of this is something that you may want to discuss with CA Services in an architectural and process review.
There isn't a "one answer fits all" scenario for CA Process Automation, but I count that under the advantage column