UI/UX is often overlooked by front-end developers. It’s a whole different discipline, yet very close to front-end development since the front-end developer is the one that makes the design well... work.
Here are some points on why you should learn UI/UX as a front-end developer:
You’ll be able to spot non-obvious design mistakes
Since you are the one that implements the design, you will spot mistakes in the development process. So, you can discuss this mistake with the designer and figure something out early.
You’ll build better-looking and usable side projects
If there is a better thing than an awesome project, it is an awesome and good-looking project.
No matter how good or useful is your project, if it's looking bad in terms of the user interface, there is a good chance that nobody will use it, let alone paying for it.
It’s the same with user experience. Even if your project is looking superb, if the user is struggling while using it, nobody will use it either.
By learning UI/UX you‘ll be able to build usable and beautiful apps.
You’ll probably worth more as a front-end developer
If you practice two or more disciplines together, you are a valuable asset for any company regardless of your field.
This is the most important part. Learning something is always fun!