As a software developer in any company, the objective is to build software for the business of the company. Building software is a complex task. Change is the only constant thing in the world. As the software progresses over the years, it needs to incorporate a lot of changes as the requirements keep on changing. We can choose to build software in many ways but we NEED to build it in such a way that the software is maintainable and extensible over the years easily. 

Most of the software developer interviews have Data Structures & Algorithms round(s). But it is not often for software developers to USE these data structures and algorithms in the real world while building software. (If you are an experienced candidate, can you recall when was the last time you used the Dijkstra algorithm? If you are fresher, ask any of your senior colleagues the same question. )

On the other hand, Low-Level Design is a must for building a piece of software. As LLD focuses on how to build software from a set of requirements, how different components interact with each other, what responsibilities each component has etc, it is a vital exercise to be done before actually writing the CODE. Writing code is the easiest part of building software if we have the designs already in place. Maintaining and Extending software will be easy if designs are well thought of.

As we can see from the above picture, Maintenance takes almost 70% of the effort in the software development LIFE cycle. So the software should be easily maintainable and extensible. LLD plays an important role in the design of software and hence directly contributes to the maintainability and extensibility of software.