Purpose of XPCservices and Agent related to an Application
XPC Services
From Apple's Creating XPC Services:

There are two main reasons to use XPC services: privilege separation and stability.
Stability:
Let’s face it; applications sometimes crash. We don’t want it to happen, but it does anyway. Often, certain parts of an application are more prone to crashes than others. For example, the stability of any application with a plug-in API is inherently at the mercy of the authors of plug-ins.
When one part of an application is more at risk for crashes, it can be useful to separate out the potentially unstable functionality from the core of the application. This separation lets you ensure that that, if it crashes, the rest of the application is not affected.
Privilege Separation:
Modern applications increasingly rely on untrusted data, such as web pages, files sent by email, and so on. This represents a growing attack vector for viruses and other malware.
With traditional applications, if an application becomes compromised through a buffer overflow or other security vulnerability, the attacker gains the ability to do anything that the user can do. To mitigate this risk, Mac OS X provides sandboxing—limiting what types of operations a process can perform.
In a sandboxed environment, you can further increase security with privilege separation—dividing an application into smaller pieces that are responsible for a part of the application’s behavior. This allows each piece to have a more restrictive sandbox than the application as a whole would require.
Other mechanisms for dividing an application into smaller parts, such as NSTask and posix_spawn, do not let you put each part of the application in its own sandbox, so it is not possible to use them to implement privilege separation. Each XPC service has its own sandbox, so XPC services can make it easier to implement proper privilege separation.
For more information about sandboxing, see App Sandbox Design Guide.