Have you ever thought about what makes an app so great? Think about it for a second... is it the colors, the font, the complexity? Now, think about a really bad app. What makes it so horrible? Is it the colors, the font, the complexity?
All these factors (plus more) are what make some apps good or bad. It's the way they were designed that makes them stand out to us.
Design includes colors, fonts, typography, graphics, backgrounds, and essentially everything that makes up an app besides the actual coding-- but even that has a major part in the whole design. The design's purpose is to grab the users attention and to make them feel good about using the app.
Design is everything when it comes to an app. It dictates the user's experience in a tremendous way. Good designing and user experience of an app are a reflection of your branding and business. It needs to reflect your theme (both from a values standpoint and a cohesive design standpoint). If a user has a poor experience with your app then it could reflect badly on the brand or the company.
No matter what the app is used for you always need to think about the end user -- what will they be using it for, how will they use it, when/where will they use it. Having a human-centered design can help you hone in on what kind of experience the user will have and what you can create for them.
Check out these examples for good and bad designs: Bad Design vs. Good Design: 5 Examples We can Learn From
So, design is not just how it looks; it's a lot more than that. It's about what kind of experience you can give to those that use it.