DLLImport

Question

Hi,
I have been experimenting with using DLL Import on user32.dll and kernel32.dll I have successfully put together my own way of working with the MessageBox as an experiment to learning the DLLs.

Could you enlighten me on what I could do or what advantages I could gain by working with such DLLs ?

also I would like to know what other DLLs I can work with.

NOTE:
Using C# 4.0