Calling app from subprocess.call with arguments

I'm a beginner in Python, and I've been trying to call a command line app, but it fails:

>>> import subprocess as s
>>> s.call("gpio -g read 17")
Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
    File "/usr/lib/python2.6/subprocess.py", line 470, in call
        return Popen(*popenargs, **kwargs).wait()
    File "/usr/lib/python2.6/subprocess.py", line 623, in __init__
        errread, errwrite)
    File "/usr/lib/python2.6/subprocess.py", line 1141, in _execute_child
        raise child_exception
OSError: [Errno 2] No such file or directory

But then if I add shell=True it all starts working. Can someone explain why?

>>> import subprocess as s
>>> s.call("gpio -g read 17", shell=True)
>>> 0