As a js dev for 6 years now, and having conducted hundreds of interviews, I would say it is not necessary for a junior developer and most of the things they'd be doing.

However, when it comes to a mid or senior level engineer, it's important not to just make things work, but to understand the how and why in order to build performant applications at scale.

So to answer your question, no it's not a necessity to build applications. Yes, it is if you want to excel at building applications.