إقتباس :What are the reasons for a Client Profile?
Although the NET4 Client Profile is smaller and faster to install compared to the NET4 Full Framework (NET4 is 7-8MB smaller in size than the Full Framework), the main reasons are not just better deployment size/time but also:

Reduce the Framework deployment failures.
By keeping ASP.Net and other components (that are mostly needed for servers) out of the Client Profile, we can reduce the attack surface and the number of future servicing events which may be caused by server component (such as ASP.Net) and are not needed for desktop scenarios.
Making NET4 Client Profile available on Windows Update will make sure that most desktop machines will include NET4 Client Profile over time and apps that target the Client Profile will not need to carry or install the Framework which will improve the overall deployment experience.
Enable us to add features and grow the size of overall Framework in future versions but still have a smaller core.

إقتباس :When to use NET4 Client Profile and when to use NET4 Full Framework?
NET4 Client Profile:
Always target NET4 Client Profile for all your client desktop applications (including Windows Forms and WPF apps).

NET4 Full framework:
Target NET4 Full only if the features or assemblies that your app need are not included in the Client Profile. This includes:

If you are building Server apps. Such as:
ASP.Net apps
Server-side ASMX based web services
If you use legacy client scenarios. Such as:
Use System.Data.OracleClient.dll which is deprecated in NET4 and not included in the Client Profile.
Use legacy Windows Workflow Foundation 3.0 or 3.5 (WF3.0 , WF3.5)
If you targeting developer scenarios and need tool such as MSBuild or need access to design assemblies such as System.Design.dll