Twitter like any other social site does advertising as well as target ads. When I log into my banking account I get ads for local banks, etc.
The more you use it and the longer you log in for, the more revenue. So your feed will be filled with posts similar to ones you've read in the hope you'll click.
When you follow someone all their posts appear in your timeline.
Follow for example 200 people who post twice a day and your feed will have:
400 Tweets to read through, plus ads and potential Tweets of a similar nature as well as recommended people.