What is `isPlaceholder` for? is it required to always check it in table headers?

I see it on every example but its purpose is not clear. All I can find is this: https://tanstack.com/table/v8/docs/api/core/header#isplaceholder

Header | TanStack Table Docs

These are core options and API properties for all headers. More options and API properties may be available for other table features. Header API