Know a full stack, even if you don’t work it every day
A very few brave developers still customize every aspect of their sites and apps, from polishing interface design on the front end to optimizing database connections on the back end. Many more people choose standard sets of tools — the LAMP stack or MEAN stack — and hope that they can stay within the lines enough to get help. Building effective web systems, though, means understanding how the layers of applications interact and their communications styles. At the very least, it means understanding the layers with which your work interacts.
This reflects my technical skills philosophy pretty succinctly although I have definitely grown to prefer the front end work.
The analogy that I use is that I prefer finish carpentry over framing or plumbing because, although both the other disciplines are important, the mainly interacts with the visible surface of the thing.