If one wants to write something in English, they need vocabulary to use in their writing. Therefore, it is essential to have enough amount of vocabulary in one's head.
But it also depends on what they want to write about. If one needs to write an academic paper they need to know a range of sophisticated and highly advanced English vocabulary, let alone using them appropriately. But if one wants to just write about their daily life they need not have as complicated and advanced as the ones mentio