title: Android: when is putExtra() and getExtra() used?
body: <p>The internet gave lots of example of HOW we can use putextra and getextra. However, I wish to ask: when is  putextra / getextra being used and are there any best practices I should know. If there's a list that I'm not aware of, please be kind to point it out. Please don't give me the syntax as I already succeeded creating an android app and know how this can be done. Can someone help?</p>