If you buy smartthings, you don’t need the Zstick. Smartthings would act as the Z wave controller and any other Aeotec devices you buy like sensors or relays would be controlled directly by smartthings.
A few people in the community do get an aeotec stick and add it as a “secondary controller“ so that they can use its mapping utilities, but that’s unusual and not something most people will need or want.
I believe if you want to update the firmware on your Aeotech devices at some point you’re going to need the Zstick as the ST hub does not (at this time) permit over the air updating of them.
But as JD said, for just simple functionality, you do not need the Zstick if you have a ST hub.