Why did Defoe write his essay "On the Education of Women"?

Daniel Defoe wrote this essay to promote the idea that
women should be educated.  That much is fairly obvious.  What is interesting, though, is
why he thought women should be educated.  Defoe believed
that women should be educated so that they could be better wives and companions for
men.

In modern times, we believe (in the West, at least)
that women should be able to go out and get jobs and contribute to the economy.  They
should essentially be the same as men in economic terms.  But Defoe did not make this
argument.  Instead, he argues that women would simply be more fun to be around if they
were educated.

As an example of this, look at what he
thinks they should study.  He is not advocating that they study math and science. 
Instead, he says that they should be taught

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... in particular, Music and Dancing; which it
would be cruelty to bar the sex of, because they are their darlings. But besides this,
they should be taught languages, as particularly French and
Italian...

He does go on to
say that they should read history books, but there is no talk of more practical
subjects.  Instead, the idea is that they should be more "cultured" so as to be more
interesting.

Later in the essay, we see Defoe explicitly
reject the idea of women participating in the same sorts of things that men do.  He says
(I have added the bold letters)

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Not that I am for exalting the female government
in the least: but, in short, I would have men take women for companions,
and educate them to be fit for it. A woman of sense and breeding will
scorn as much to encroach upon the prerogative of
man...

From this, it is clear
that Defoe's reasons for wanting women educated are not modern ones.  Instead, he simply
wants women to be more pleasant companions.  He believes that this will happen if they
are more educated.