First of all, it's important that we define what soft skills are. They can be defined as any behavioral skill that can help someone interact effectively with other people. In an industry where everyone work in teams remotely or physically, you can understand how that would be essential. As an example, we can mention communication and leadership skills.
The classic developer stereotype is the guy that always stays in his corner, typing out nerdy stuff. That narrative is the exact opposite of the reality. Software developers most of the time work in Agile teams where communication and team spirit are essential. They are also implicated in the process of product design, and have frequent contact with other people such as designers, customers or stakeholders. Thus it’s very important for them to know how to effectively and harmoniously interact with other people.
The software development community also includes freelancers and indie hackers. Those developers help businesses reach their goals or create their own products. Even if they don't most of the time work in a team, soft skills are even more relevant to them. They constantly interact with customers and not having good work ethic, communication skills and adaptability can be fatal for their business.
The truth is that customers and investors don’t care about the technicalities behind a product. If one wants to be able to sell or promote his/her work, good "people skills" is decisive.
I recommend to start working on specific things like public speaking, assertive communication and active listening. I am still working on these skills but books like Public Speaking Skills For Dummies or Messages: The Communications Skills Book helped me a lot on my journey to developing my soft skills as a software developer.
With that said, I hope you’ll be more aware of these skills because they are a key thing if you want to have a successful and brilliant career.