What does sys.intern() do and when should it be used?

From the Python 3 documentation:

sys.intern(string)

Enter string in the table of “interned” strings and return the
interned string – which is string itself or a copy. Interning strings
is useful to gain a little performance on dictionary lookup – if the
keys in a dictionary are interned, and the lookup key is interned, the
key comparisons (after hashing) can be done by a pointer compare
instead of a string compare. Normally, the names used in Python
programs are automatically interned, and the dictionaries used to hold
module, class or instance attributes have interned keys.

Interned strings are not immortal; you must keep a reference to the
return value of intern() around to benefit from it.

Clarification:

As the documentation suggests, the sys.intern function is intended to be used for performance optimization.

The sys.intern function maintains a table of interned strings. When you attempt to intern a string, the function looks it up in the table and:

1. If the string does not exists (hasn’t been interned yet) the function saves
   it in the table and returns it from the interned strings table.

   >>> import sys
   >>> a = sys.intern('why do pangolins dream of quiche')
   >>> a
   'why do pangolins dream of quiche'
   

   In the above example, a holds the interned string. Even though it is not visible, the sys.intern function has saved the 'why do pangolins dream of quiche' string object in the interned strings table.

2. If the string exists (has been interned) the function returns it from the
   interned strings table.

   >>> b = sys.intern('why do pangolins dream of quiche')
   >>> b
   'why do pangolins dream of quiche'
   

   Even though it is not immediately visible, because the string 'why do pangolins dream of quiche' has been interned before, b holds now the same string object as a.

   >>> b is a
   True
   

   If we create the same string without using intern, we end up with two different string objects that have the same value.

   >>> c="why do pangolins dream of quiche"
   >>> c is a
   False
   >>> c is b
   False
   

By using sys.intern you ensure that you never create two string objects that have the same value—when you request the creation of a second string object with the same value as an existing string object, you receive a reference to the pre-existing string object. This way, you are saving memory. Also, string objects comparison is now very efficient because it is carried out by comparing the memory addresses of the two string objects instead of their content.