Many of us reading this are data professionals, and we likely know quite a bit about SQL in general. We may use T-SQL, specifically, with SQL Server, but many of the skills we have would be portable to other dialects, such as those use in Oracle, PostgreSQL, etc. We'd certainly need to brush up on best practices and which language constructs are better suited for a specific platform, but most of the knowledge transfers.
I ran across an interesting post on the value of SQL in a career, mostly for someone that might be moving into the data science or machine learning/AI type role. The post notes that lots of these job descriptions mention SQL, but focus quite a bit on R, Python, modeling, etc. In this case, SQL matters, and I'd agree with the author that it matters a lot. Most of the work in those fields is data prep, and SQL makes this much easier at scale than other languages.
What about those of us that have been working with SQL Server (or some other platform) for awhile? Is learning more about SQL a good investment in our career? In most cases, does our boss even know if we have mediocre or amazing T-SQL skills? Do they care?
They may not, but I think they should care, and more importantly, you may care. When you know more about the language and how to structure queries to solve problems, you'll work quicker. You will write code that performs better, resulting in a lower workload on the instances. Your code will last longer, have fewer bugs, and co-workers will trust your work.
The more you practice with code, the more you solve new problems and learn what works well and what doesn't, the better you will be at your job. The time you put into learning to write better queries will pay back with less stress and more time for other tasks. While your boss might not notice your code is better, they certainly will see you as more capable, relaxed, and trustworthy. All good impressions to make at review time.