How does learning about the historical period help you understand what you read?

Societies and contexts change, as does language, so it can be very easy to misunderstand a text if we don't understand basic facts about the historical period in which it was written. It is easy to superimpose the values of one's own culture on a society that may be very different.
Jane Austen's novels are a good case in point. To understand them, it is important to know some facts about English society in the early nineteenth century. It is, for example, significant that ladies (upperclass woman) were not expected to work for money, and that almost all career paths were closed to them. This helps us understand the importance the women attach to marriage. Women were extremely concerned with finding a husband because their society gave them almost no other options for getting ahead in life, unlike in today's world.
Likewise, it is important to understand how humiliating it was for a lady to have to become a governess. You lost status in society the minute you accepted money for work. You were what was called "sunk" in society. Living in a grand house and teaching a few children may sound like a cushy post, but it was humiliating. A parallel today would be having to become a live in maid for your friend at low wages and take orders from her. Thus, when Jane Fairfax in Austen's Emma resists becoming a governess and refers to it as a form of slavery, she is not being lazy or whiny: she is being realistic about what is ahead of her.
Knowing something about the language of a historical period helps too. For example, when Jane Austen's heroes "make violent love" to a heroine, this is not a physical encounter—it means that the male is using very strong and unmistakable love language. If we don't know this, we can easily misinterpret what is happening.