So what is a thread? You can think of threads in Java programming just like threads in a story. In one thread of a story, we have the primary character battling the enemy on the front line, and in another thread, the soldier's family are getting by, day to day. Of course, a story doesn't have to have just two threads. We could introduce a third thread. Perhaps the story also tells of the politicians and military commanders making decisions. These decisions subtly, or not so subtly, affect what happens in the other threads.
Threads in programming are just like this. We create parts/threads in our program and they control different aspects for us. We introduce threads to represent these different aspects because of the following reasons:
They make sense from an organizational point of view
They are a proven way of structuring a program that works
The nature of the system we are working on forces us to use them
In Android, we use threads for all of these reasons simultaneously...