According to Twitter, “Through our tests in Brazil, Italy, India, and South Korea, we learned Fleets helped people feel more comfortable joining the conversation – we saw people with Fleets talk more on Twitter. Those new to Twitter found Fleets to be an easier way to share what’s on their mind. Because they disappear from view after a day, Fleets helped people feel more comfortable sharing personal and casual thoughts, opinions, and feelings.”
https://twitter.com/Twitter/status/1328684389388185600
Fleets are similar to Stories in which they are designed to be temporary and will be deleted after 24 hours. This means that if you wanted to post something on Twitter without it being permanently part of your timeline, or something that you might not typically share on your timeline, then this could be an option.
According to the company, the feature is beginning to roll out to all users, so if you don’t see it yet, don’t worry as it should eventually make its way to you.