Trade allows each person to specialize in the activities she does best.
Why exactly she (and not he)? Don't we use they in such cases (unspecified sex)?
There are lots of opinions about how to refer to a single individual of unspecified sex.
When people sat down to write formal grammars of the English language (in the 18th and 19th centuries, if memory serves), they didn't rely on how people actually wrote and spoke as much as they did on trying to make things logical and consistent - or at least, what they thought of as logical and consistent. Using similar rules to Latin was also a major goal.
These grammarians decided that they is always semantically plural, but blithely stated that there was no need for a neutral personal pronoun because you could just use he for any individual unless you knew that the person was female. Some people still use this, and it's common in legislation of the UK Parliament.
Of course, a lot of people got upset about that in the 20th century, we started getting he/she, he or she, and so on. A lot of people found that unwieldy, but they had to deal with teachers, editors, commentators and so on who'd learned grammar based on the 19th century works.
Prior to the work of these grammarians, they as a semantically singular (but grammatically plural) personal pronoun was well-attested. Shakespeare used it. So now, a lot of people are using it again - and a lot of other people get stressed and upset about it, declaring it wrong. However, it's advocated by the British Council in their materials for learning English.
Finally, some people decided that using just one gender pronoun to refer to a person of unknown gender was fine, but to make a point they would switch which one it was, and use she in the way that the 19th century grammarians said they should use he.
I won't bother getting onto the more fringe alternative pronouns that some people advocate, as they are far from mainstream.
Actually, past generations were taught to default to the masculine pronoun he. This is still often found in written English, especially in written instructions. You can imagine how much longer a text would be if the writer had to write "he or she / his or hers" every single time they referred to an unspecified person, especially in instructions where brevity is preferable.
Some more modern texts may begin by explaining their choice of pronoun, for example:
In these instructions I will refer to the end user by the pronoun "he". The instructions equally apply if the end user is female.
An example of this is the 1999 child-rearing manual The Contented Little Baby Book in which the author refers to your baby throughout as "he".
In today's climate where some issues surrounding gender can be controversial, you may find that indeterminate pronouns such as "they" are preferred.
As to why your text uses "she" - I can only assume that either:
An example of this latter would be:
I was talking to God the other day, and she said "yo what's up?"
This example is for humorous effect, as it challenges the perception of most people that God is male in a irreverant way.