AppCmd.exe and Powershell

I have just started going through some tutorials of Powershell - for I want to write scripts to create website/virtual dir/appPool
However I am not very clear about few things - 

• What is Powershell SnapIn
• AppCmd.exe and Powershell both can be used for IIS 7.0 administration tasks.
  Why Microsoft provides two different tools(AppCmd and Powershell) with almost same features?

Can please share your knowledge to clarify these?

Thanks a lot!