Failing to look at your design from the point of view of the people that are actually going to be using it is catastrophic. End users that can not comprehend the design, understand how it works, and get the end result that they are looking for, will eventually abandon their tasks. Your design should help your end users complete their task in the easiest way possible.
For example, if you want website visitors to purchase an item, it should be really obvious how they can purchase that item: big add to cart buttons, easy checkout paths, and minimal friction to complete payment. If your design does not guide the user and make it simple for them to perform their task, your conversions suffer. You’ll have more people that fail to complete their task because they abandoned the process. The interface was too big of a barrier – they couldn’t figure it out or it wasn’t obvious to them what they were supposed to do.
A big design trend recently has been minimalistic. Simplify the UX so that it’s completely obvious exactly what the end user is supposed to be doing. Guide them down the exact path you want them to take by removing all the distracting noise. Take out the bells and whistles and make it function extremely simply. Apple has been doing this for years.
On Reddit, there was a thread that went around to try and get designers to create the worst volume control in the world. The challenge was, take something extremely simple (like the volume control) and make it extremely challenging to use.
The results are priceless, and it showcases exactly why UX is extremely important.