What is the purpose of a topic sentence in a paragraph?

To interest the reader and inform them a little bit on what that paragraph or whole essay is going to be about

because each is self-sufficient.  

animal farm, written by george orwell, is a representation of russia after the revolution that led stalin to take over power.

in this excerpt, the central idea seems to be that the animals (a representation of society), when they believe to be working for a greater good and equality, are obedient and productive. they were convinced that, after expelling the owner (jones) from the farm, they would live in harmony and be fairly treated.

as we know, the pigs become the leaders and grow more selfish and dictatorial with time. in the end, they look and act in a way so similar to humans that it is impossible to tell the difference. the ideals of equality are shattered, and the farm turns out to be worse than it was before. the animals are manipulated with false propaganda and forced to work in order to enrich the pigs.