What is the use of join() in Python threading?

Straight from the docs

join([timeout]) Wait until the thread terminates. This blocks the calling thread until the thread whose join() method is called terminates – either normally or through an unhandled exception – or until the optional timeout occurs.

This means that the main thread which spawns t and d, waits for t to finish until it finishes.

Depending on the logic your program employs, you may want to wait until a thread finishes before your main thread continues.

Also from the docs:

A thread can be flagged as a “daemon thread”. The significance of this flag is that the entire Python program exits when only daemon threads are left.

A simple example, say we have this:

def non_daemon():
    time.sleep(5)
    print 'Test non-daemon'

t = threading.Thread(name='non-daemon', target=non_daemon)

t.start()

Which finishes with:

print 'Test one'
t.join()
print 'Test two'

This will output:

Test one
Test non-daemon
Test two

Here the master thread explicitly waits for the t thread to finish until it calls print the second time.

Alternatively if we had this:

print 'Test one'
print 'Test two'
t.join()

We'll get this output:

Test one
Test two
Test non-daemon

Here we do our job in the main thread and then we wait for the t thread to finish. In this case we might even remove the explicit joining t.join() and the program will implicitly wait for t to finish.

A somewhat clumsy ascii-art to demonstrate the mechanism: The join() is presumably called by the main-thread. It could also be called by another thread, but would needlessly complicate the diagram.

join-calling should be placed in the track of the main-thread, but to express thread-relation and keep it as simple as possible, I choose to place it in the child-thread instead.

without join:
+---+---+------------------                     main-thread
    |   |
    |   +...........                            child-thread(short)
    +..................................         child-thread(long)

with join
+---+---+------------------***********+###      main-thread
    |   |                             |
    |   +...........join()            |         child-thread(short)
    +......................join()......         child-thread(long)

with join and daemon thread
+-+--+---+------------------***********+###     parent-thread
  |  |   |                             |
  |  |   +...........join()            |        child-thread(short)
  |  +......................join()......        child-thread(long)
  +,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,     child-thread(long + daemonized)

'-' main-thread/parent-thread/main-program execution
'.' child-thread execution
'#' optional parent-thread execution after join()-blocked parent-thread could 
    continue
'*' main-thread 'sleeping' in join-method, waiting for child-thread to finish
',' daemonized thread - 'ignores' lifetime of other threads;
    terminates when main-programs exits; is normally meant for 
    join-independent tasks

So the reason you don't see any changes is because your main-thread does nothing after your join. You could say join is (only) relevant for the execution-flow of the main-thread.

If, for example, you want to concurrently download a bunch of pages to concatenate them into a single large page, you may start concurrent downloads using threads, but need to wait until the last page/thread is finished before you start assembling a single page out of many. That's when you use join().