Obviously it depends on the product/project, but the backend (data modeling, state management, data locality, etc) is crucial to how the product works and UX. The most common gaps I've seen are ineffective reasoning about the state of the user over time, permutations of different logical states, and consistency across surfaces owned by different teams. A good BE product engineer can get their head around the whole problem space and reason about the UX holistically in a way that can provide a huge amount of value to designers and product managers in interactive sessions. If BE does not get a seat at the table you often end up with non-sensical PRDs, designs missing critical details, or worse visual typos end up being interpreted as requirements.

Of course many FE engineers have the chops to solve for these things, as well as PMs and designers with technical background, deep domain expertise, and other qualifications that can make up for these gaps. I came up professionally in the early web days and building startups where it was normal to wear many hats, so it's more about skillsets than rigid titles and role definitions. I've just seen a very common gap that a lot of ostensibly professional proddev folks, even at top companies, have borderline magical thinking about the deeper layers of the product stack.