The thunderbolt 3 (usb-c) to thunderbolt 2 adapter lets you connect thunderbolt and thunderbolt 2 devices — such as external hard drives and thunderbolt docks — to any of the thunderbolt 3 (usb-c) ports on your macbook pro.
As a bidirectional adapter, it can also connect new thunderbolt 3 devices to a mac with a thunderbolt or thunderbolt 2 port and macos sierra or later.
In addition, it can be used to connect thunderbolt-enabled displays — such as the apple thunderbolt display and lg thunderbolt 2 displays — to any of the thunderbolt 3 (usb-c) ports on your macbook pro.