A simple question: Why do we need Android SDK installed in our system if we want to automate our anroid application?

Hi,
A have a very simple question:
Why do we need Android SDK installed in our system if we want to automate our anroid application?
I used to think that as adb is a part of SDK, so this is the major component why we need SDK to be installed into the system.
But someone gave me some other reason, so it’s just something that i want to know. What all things we need from SDK to automate any app?