Understanding When to Use Branches

As a git noobie I am beginning get get my head around git's version control philosophy. I am now trying to understand the purposes of branches or rather when to use them. In my case I have a Java application under version control with git. I am planning to port it into a mobile app. Is that an appropriate use of branches or should it be created as a new repository? What is the relationship between the same source code in different branches? Do changes to code in one branch get ported to another branch somehow or do all changes then have to be made twice? The documentation tells how to branch but not the general philosophy behind it from a best practices standpoint. Thanks in advance for any insight.