Means that someone can read as usual, or delve for the full series of tweets. They could provide an indication to the writer of what percentage of people read to which depth, for example.
It could work well for presenting news articles (sentence or paragraph at a time), photo galleries/essays, slidedecks, stories, advertising, etc. Using a news article as an example, some people will be interested in just the headline, others in the first three bits, a few in the full story to its end.