Opencart modules are used for some special functionality you want in your store, which are not provided by default.
Opencart modules are used to provide more functionality to your store. It can be a theme or related to shipping, payment, checkout, blogs etc. Which features are not provided by default.
These modules can be paid as well as free too. Depending upon your requirement you can buy and add it to your store.
The modules are developed for front-end users and for admin user also.
The site for purchasing/downloading opencart module is Opencart Extensions