How to decide when it's worth investing the time to learn something new
First of all, good luck on your PhD! 

The way I was explained it, the point of a PhD is to become super good at something. To do so, a shallow understanding of lots of things is oftentimes less useful than a deep understanding of one thing. This is just practicality: you need to graduate in X years with a PhD certificate. I would say 80/20 ratio in favor of deeper understanding of a topic vs reading something new, at least. 

I would attend seminars anyway to keep abreast of what’s going on, and to learn how to speak about one’s work.