Why is _id needed?

Question

MongoDB

I know that this is the unique id of any document from any collection. I see the point in using it in the document of the logged in user to store it in the session and identify it. There is a sense to yuzat it for simplification of search in a DB.
But, let's say I have a collection of VK users, I can identify a user by id in VK. Does it make sense to use objectId here? With the condition that I will not have the "/vk/get?id=..." route, where I simply give the user id and get the user I need from my database, I will receive data about the user from VK only using the session .
And still about simplification of search on a DB. Is it safe to give the objectId to the frontend? For me, this immediately begins to be associated with a vulnerability :( Here, too, with the condition that using the session and checking that the document being searched belongs to the user from the session.