Difference between Popen and open() for reading a file

[J]

I was reading something from a code review a little while ago and saw
something that's got my curiosity up...

Say I had a file, foo.txt that I wanted to read from, only one time
and only read.

So what's the difference between this:

mylist = Popen(["cat","foo.txt"], stdout=PIPE).communicate()[0].splitlines()

and this:

f = open('foo.txt')
mylist = f.readlines()
f.close

Is there a reason why you would not use subprocess.Popen for this,
other than just not relying on external programs to perfrom the task?

what if that file only has one line in it, and that's all you're
interested in, and the file is guaranteed to only have that one line
it it?

For example:

say foo.txt contained only the number 123456789

what's wrong with doing this:

my_int = int(commands.getoutput('cat foo.txt')

or via the subprocess.Popen method mentioned above?