You don’t need a mock viva. You need practice for the viva, a rehearsal space to think and respond like you might do in the viva. You don’t need a mock viva, it’s just one of the ways that you could rehearse.
You don’t need a mock viva, it’s just supposed to be like a real viva, with substitute examiners who will be well-placed to ask you relevant, helpful questions to give you a sense of kind of discussion that arises in the viva. You don’t need a mock viva, there are lots of ways to get help, but given that it’s supposed to be like the viva it could be a really useful opportunity if you have the chance.
You don’t need a mock viva, you need to be ready: a mock is a means to an end, not the end itself. There are other things you could do that would help.
But if you have the chance, a mock viva could be exactly what you need. A small, self-contained piece of preparation. A boost to confidence, to awareness, to expectations. A chance to rehearse and build. A chance to get ready.
You don’t need it. But it might help a lot if you have it.