Some tweets just aren't meant to last forever. If you're posting something that won't be relevant after a certain period of time, Spirit can delete it from your account using a hashtag.
Once you connect Spirit to your Twitter account, any tweet with a hashtag indicating an amount of time (think #30m, #10h, #5d) will be deleted automatically when it expires. This is great if you're running giveaways or posting time-sensitive information to your company's account, but it can be useful for personal accounts too. For example, maybe you don't want your thoughts on Miley Cyrus to be visible to the public five years later. Once you install Spirit, you never have to think about it again until you use it, so it's definitely worth checking out.