Well, let's see, your question is basically referring to historical context here. Elizabeth Barrett Browning is usually found in "The Victorian Age" unit of any British Literature text. Quite simply, she lived in the 1800s (born in 1806, I believe). She lived in England and was fairly well off, from what I understand. She is famous for her sonnets, ... sonnets that she never really wanted to publish but was pressed to publish. She became famous for said sonnets.
Now, it's the "why" of this particular sonnet that gets really interesting. You see, Elizabeth and Robert Browning (another famous English poet) began a relationship through letters and fell in love when they met. It is pretty safe to say that this poem is written about him. In addition, it makes reference both to their love and their faith (both significant in their lives).
To get further into the "why" we can delve a little bit deeper into the role of women in the 1800s. Women at this time felt some social pressure to be a wife and mother only. In fact, they felt obligated to do so because it upped their class status. Browning, like many women, didn't receive formal education. (In my opinion, this makes the prowess of her sonnets even more unbelievable.) In the US, this idea of submissive and obedient wives formed what was called the doctrine of "femme convert." Generally, it's everything we've worked against the last hundred years or so: husbands had complete control both legally and socially. I think it's pretty safe to say that Browning was fairly accepting of her role as wife and mother. I would guess that her reluctance to publish her poems had something to do with these pre-feminist factors. And yet, she is a respected and published author, isn't she. Perhaps she's having the last laugh.