A Public URL Shortener is a marketing tool used to turn long messy URLs into neat, clean, smart, short links which redirect visitors to the actual long URL when clicked. 

This is generally used due to its flexibility and available functionality to track link clicks and other visitor statistics. Some Link Shorteners like Publ.cc allows you to use your own custom domain to fully brand your shortened links, that way your customers knows the short link is from your brand they know. Also giving you access to seing behind the scenes link statistics such as how many clicks your link is getting, where are these clicks coming from (Instagram,  Twitter,  website,  email, What country,..ect.) this way you can make more prudent decisions for reaching your target audience effectively.