Differences between use of os.path.join and os.sep concatenation

I am trying to figure out if it is better to use:

os.path.join(str1, str2)

or:

str1 + os.sep + str2

Profiling with timeit I found that, as expected, concatenation is faster:

%timeit 'playground' + os.sep + 'Text'
10000000 loops, best of 3: 139 ns per loop

%timeit os.path.join('playground', 'Text')
1000000 loops, best of 3: 830 ns per loop

So my question is, since concatenation is also shorter, is there a reason to use os.path.join(()?

Thanks