Hi and welcome to Daz Studio,
The short answer is no, you do not need any other applications to use Daz Studio. Daz Studio is a posing and animation program and as such you create scenes by using purchased or free content made by other people. You’re a photographer in a photography studio so to speak.
DAZ the company does have other softeware packages though because some people want more than to be just the photographer. Prerhaps you want to create your own clothing, buildings, vehicles and so on. For this you would need a modeling program which is exactly what Hexagon is.
Or perhaps you want to create your own environments with majestic mountains, valleys and lakes. It is towards this purpose that Bryce is used althought the experts will tell you that Bryce can do quite a bit more than just terrains. I’m still trying to get my mountains to behave!
There’s also Carrara which can do a bit of everything but it’s not free. It can create landscapes, you can model in it and you can pose and animate in it. It does have a physics engine though as well as particles (fire, smoke, etc.) and dynamic hair. Again, I cant even keep my own hair in place so I havent tried that yet.
So yeah, if you’re just getting started then don’t worry about the rest for now. I would advise you to ‘purchase’ Bryce and Hexagon if they are still free just so that you don’t miss out on them later on.