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
... 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)
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.