At times, you need to reuse content, similar to templates again and again. For that, you can write your tweet once and can plug it every time you want to use. 

Use case

Suppose you sell a software. You write educational content and how-to guides in form of threads from your page and want to plug the sign-up page of your software at the end of each such thread.

Plugs could be used to attach a link to a blog, affiliate product, software or any other content that you would share again and again. Most influencers use this feature to sell an affiliate product.