Great question. Transitional sentences make a connection between the previous sentence, and the sentence following.
In your first example, your initial topic is discussing Chinese Novels. The transitional sentence used there is a quote by Steve Jobs, which allows for the topic to move from discussing the Chinese Novels, to Steve Jobs' biography.
The reason that particular sentence is considered the transitional sentence is because it allows for the topics to change in a fluid motion from one idea to the next, without actually interrupting the flow of what is discussed in the reading itself.
In your second example, the transitional sentence is the "there are challenges of course". The reason that this is a transitional sentence is like before. If we look at the topics that are discussed between the first section and the second, they are independent of each other. However, with that transitional sentence, it allows for the topics to join together into one cohesive discussion or topic.
Does this help?