For many web designers, matters of code standards and style are often left in the hands of their fellow developers. You give developers their deserved space to wield their magic to turn your designs into functional products that people love to use. The team's coding standards are often the last thing on your mind. So why should you and other designers care?
Tech debt steals time away from new features and enhancements
The next time you are ready to go with a newly designed feature, you might find its development gets stuck behind a mountain of tech debt. Addressing tech debt at a reasonable pace throughout the life of a product can prevent it from piling up to a point where feature work gets put on hold. Better yet, if these issues are caught during initial development, that tech debt might be avoided in the first place.
It can be in your best interest to encourage developers to address issues in the code as they arise, rather than focusing only on design. Failing to fix these issues will cost you in future sprints.
Get better at your own craft
Understanding what it takes to build great software ultimately makes you a better designer. Whether you want to improve how you write your own front-end code if that's your thing or gain a better appreciation of the time and effort it takes to build products, your practical experience will allow you gauge future work and effort.
A lot of similarities exist between coding standards and design standards. Just as you wouldn't want a designer to go off-brand, developers have the same standards they want to maintain in their code.
Code should look good too
Steve Jobs was famously renowned for being picky about both the externals as well as the internals of Apple products. Your beautiful UI work can only go so far if it isn't built on a solid foundation of code. Pushing through a feature without appreciating the work to build it properly can lead to bugs later. The right amount of time and attention at the initial development cycle can reduce these mistakes.
Designers can be champions for well written code. Just as you appreciate the details in designing great user interfaces and experiences, appreciation of the time and effort it takes to build those properly goes a long way.