The Mystery Box, when opened, provides a special bonus. For 30 minutes, you will have the ability to find the Mythical Pokémon Meltan and catch it. It spawns on average of one Meltan every 1.5 minutes.
When the time runs out, Meltan will stop appearing in the wild. However, you can re-open the box after waiting for 7 days. After that time, send another Pokémon to Let's Go and you'll be able to open the box once more.
Unique Special Research will also take place once opened focusing on Meltan. This opens up this Winter