Illustrate means to clarify or to make a clear statement by providing visual examples. Illustrations can be important in the user interface depending on its mission and effectiveness, such as loading screens with illustrations helping the user to remain attracted to the application and keep them entertained while waiting looking at the loading time.
Illustrations play a key role in telling the story of the brand more as they are a visual side of information that is picked up and understood faster than written language.
icons allow us to see how to use the interface and how we should be using it, icons explains what we can do e.g. Heart icon to favourite an item.
Thematic illustrations refer to the main topic that the brand or application is trying to portray e.g. a banking app will have illustrations relating to currency, banking or more. As well as meditation applications that use illustrations to create a theme and a vibe related to the topic.
(Dribbble, MushroomMin)