Thank you for these interesting questions:

1. It’s mostly because in real life we don’t really have time nor budget to run one more day of user tests, one week after the Sprint. If you can do it, I think it’s awesome! (in our case it’s more about fixing the clear painpoints of the Prototype, not creating anything new)
2. Numbers. You will get real data and be able to measure how many people visit, buy, interact, how much time they spend… These data tell the truth. Sometimes during the 5 user interviews, testers will try to please you, or make you feel good.
   - Would you buy this product?
   - of course!
   - Would you buy it for $49?
   - Sure! I’d love to
   - OK, so here is the Paypal form ->
   - Wait… I need to buy for real?? I thought it was just a test!!