Umbrellas
Viva prep is a bit like carrying an umbrella on an uncertain day.
You look out the window. There are clouds. They’re grey but not dark grey. It might rain but it might not. You could leave the umbrella and take a chance or pick it up, carry it with you, make the effort and be sure that you’re covered.
After years of work, learning, results, development and growth, it’s reasonable to assume that a PhD candidate is good. Viva prep then helps ensure that a candidate is good for the particular challenge they’ll find at the viva. They might be fine, but prep will make sure that they are.
Carrying an umbrella is not a great effort to be sure of keeping dry. Neither is viva prep so great an undertaking to be sure of doing well.
You might be fine without it, but why take the risk?