Modern user interfaces allow people to interact with machines in a natural way within the context they are built upon. This is achieved through a wide variety of widgets (actors) designed with a specific purpose.
Having a well-cared interaction is also very important to immerse the user in your game experience.
Imagine the main menu for a shooter game with horror tinges where each and every detail counts: slightly lighted buttons with images, a nice gloomy font type for the main title, and a blunt gunshot sound when selecting the play option. The story doesn't end here because you must expand this environment to the settings screen where you might want to allow the user to change the volume through a custom-themed slider or set mutually-exclusive graphics options.
Libgdx provides you with a long list of customizable built-in widgets to save your valuable time, focusing your efforts on design. However, they are generic, so don't hesitate to implement your own version...