One of the biggest advantages of using a PCL (Portable Class Library) is that as the name suggests, the library contains only code that is guaranteed to run on all .NET platforms. You write it once, you deploy it once, and everything can use it. This differs greatly with a shared library that has no guarantee of running on all .NET platforms.
Note
For more on PCLs, refer to http://developer.xamarin.com/guides/cross-platform/application_fundamentals/pcl/introduction_to_portable_class_libraries/.
This may not make a great deal of sense—surely, everything should be able to run the .NET code, irrespective of whether it is running Mono, Microsoft .NET, or GNU .NET. This is not an unreasonable assumption to make, but let's think about this logically.
If you consider a piece of hardware capable of running some flavor of .NET, you cannot unreasonably think that it will have some form of storage, some form of output, and anything else you would expect to find on a computer, mobile...
