Why to use plugins
The best thing about plugins is that you don't need any specific programming knowledge in order to use them. Essentially, they are just like standard applications for iOS or Android. You can install them and enjoy the things they have to offer without knowing what's going on inside.
Furthermore, the right combination of plugins can make your site more optimized, more user-friendly, more attractive, more social-media-friendly, properly backed up, protected against spam, and ultimately, unique. Plugins really are one of the best things about WordPress.
You see, before content management systems like WordPress were popularized, there was no easy way for site owners to introduce new functionalities on their sites. Doing so always required hiring a professional programmer and investing in the whole development process. Nowadays, this is no longer the case and virtually anyone can have an impressive site without losing their shirt.