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.
If you are just building frontend applications using frameworks like React and Vue, you don't really need to know how things work under the hood. Especially with all of the new syntactic sugar introduced to JS in the last few years.
But, if you are building lower level tooling and you want to understand why things are implemented the way they are (this post being a great example) you really should understand them as best you can. A lot of design decisions in JS are influenced by these lower-level design "constraints."