For Manu, you need to reserve beforehand because these are multi-day trips and they don't leave every day. So, unless you're prepared to wait around in Cusco for days, planning-wise, it makes more sense to book beforehand.

For the Nazca flights, I had no trouble booking the night before, but that was in 2011.

Waiting around in Nazca (desert village) probably way less interesting than waiting around in Cuzco with so many sites to explore.

Was easy to book Nasca and Paracas on the spot in 2020.

Thank you. I can confirm it is still easy to book Nasca and Paracas on the spot.