Use of threading.Thread.join()

Use of threading.Thread.join()

I am new to multithreading in python and trying to learn multithreading
using threading module. I have made a very simple program of multi
threading and i am having trouble understanding the
threading.Thread.join() # function
Here is the source code of the program I have made
import threading
val = 0
def increment():
global val
print "Inside increment"
for x in range(100):
val += 1
print "val is now {} ".format(val)
thread1 = threading.Thread(target = increment, args = ())
thread2 = threading.Thread(target = increment, args = ())
thread1.start()
#thread1.join()
thread2.start()
#thread2.join()
What difference does it make if I use
thread1.join()
thread2.join()
which I have commented in the above code? I ran both the source codes(one
with comments and the one without comments , but the output is the same.