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See http://bugs.python.org/issue18861 and the discussion started here: https://mail.python.org/pipermail/python-dev/2013-November/130155.html.
Basically it allows to add context information to a traceback without changing the type of the exception.
In the following example:
 import itertools
 ', '.join(itertools.chain((str(i) for i in range(100)), [42]))
the join method itself adds context information to the TypeError:
 Traceback (most recent call last):
 File "hurz.py", line 2, in <module>
 ', '.join(itertools.chain((str(i) for i in range(100)), [42]))
 TypeError: sequence item 100: expected str instance, int found
i.e. the "sequence item 100" is context information.
However when the exception occurs higher up in the call chain, no such context information is added:
 import itertools
 def foo(x):
 return str(x+1)
 ', '.join(foo(x) for x in itertools.chain(range(100), [None]))
This gives:
 Traceback (most recent call last):
 File "hurz.py", line 6, in <module>
 ', '.join(foo(x) for x in itertools.chain(range(100), [None]))
 File "hurz.py", line 6, in <genexpr>
 ', '.join(foo(x) for x in itertools.chain(range(100), [None]))
 File "hurz.py", line 4, in foo
 return str(x+1)
 TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'
With frame annotations the traceback might look like this:
 Traceback (most recent call last):
 File "hurz.py", line 6, in <module>
 ', '.join(foo(x) for x in itertools.chain(range(100), [None]))
 File "hurz.py", line 6, in <genexpr>: sequence item 100
 ', '.join(foo(x) for x in itertools.chain(range(100), [None]))
 File "hurz.py", line 4, in foo
 return str(x+1)
 TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'