why did charles dickens use the word reaping?what does it mean?

I think this has to do with one of its definitions, "To obtain as a result of effort". When put in that context, the chapters make more sense.

"You reap what you sow," is an old saying, and one that Dickens uses in the forst two books. Book one exemplifies what the characters sow, the crops of their lives. A farmer plants his crops, what he reaps will depend on the care he's given to that which he has sown.

In Book II, Reaping, Dickens highlights the results of what the characters have sown..... Louisa'a loveless marriage sows disaster and adultery; Tom's vices lead to thievery and death; Stephen's bad marriage, his wife's drunkeness, and his love for Rachael.

What Dicken's shows us here with the use of this word is examples of the fact that everything we do has consequences, good or bad, our actions have repercussions.