The use of featured images in your posts is a very visual way to invite visitors to read your content. These images appear in your lists of posts as a preview that usually appears next to the title, depending on the theme you are using.
For your website to have a professional appearance, you need to take care of the images you publish. The quality of the images in your website is one of the most determining factors to achieve a look that attracts visitors and seduces your potential readers/customers.
In short, if your WordPress theme supports featured images, use them to improve the visual look of your WordPress site.