As an Indie Author and Publisher, we are always looking for no-cost ways to promote our work. The main way most of us do it is to follow each other and retweet each others' publications.
I have over 15k followers on Twitter, which is about average, I suppose. So when I tweet one of my books, about two or three hundred people will see the tweet. Then, if someone retweets the tweet to their followers, that is more people who will see the message who perhaps have never heard of me or my books. Some of those may be inclined to follow me. So the activity breeds more activity.
I try to always return the favor is someone retweets one of my books. Even if they are not an indie author, I will go to their site and retweet their pinned tweet, so long as it is not porn or something objectionable to my readers.
There are lots of twitter users on Goodreads. If you have a Twitter account, I will almost always follow you back. Follow me @D3NWARR3N.