It depends on the kind of application and scale. Consider a large and complex application like Discord or Figma. Past a certain point, it's hard for anyone to know how every single detail works.
You should probably be comfortable enough to work with both ends of the spectrum, but specialization allows you to do a much deeper dive into complex subjects.
A backend engineer probably has a much deeper understanding of every little nuance of their prefered database. A great backend engineer can make sure that you're getting near-optimal performance from every important query.
A frontend engineer probably knows about various UX techniques along with how to avoid unecessary reflows and repaints. A great frontend engineer can implement a UI toolkit as well as advanced techniques such as windowing.
You should probably be comfortable enough to work with both ends of the spectrum, but specialization allows you to do a much deeper dive into complex subjects.
A backend engineer probably has a much deeper understanding of every little nuance of their prefered database. A great backend engineer can make sure that you're getting near-optimal performance from every important query.
A frontend engineer probably knows about various UX techniques along with how to avoid unecessary reflows and repaints. A great frontend engineer can implement a UI toolkit as well as advanced techniques such as windowing.