Thesis submission typically comes after many hundreds of days of work: learning, development, growth and making something. You can’t get to that point without learning how to do something well.
Perfection isn’t attainable, but viva prep is a short period of time to help you get better for the particular task of the viva. You can clean up the rough edges of your memory, smooth over common stumbles in how you explain things and be more confident for meeting your examiners.
It’s a little work, but it can help a lot.