Some people use the hue bridge, some people don’t. It just depends on exactly what you need from it.
If you connect it directly to the smartthings hub, then it will work as a motion sensor for smartthings, but it will not be visible in the hue app and you cannot use it with HomeKit.
If you connect it to the hue bridge, it will be visible to the bridge. If you are using the official smartthings integration, it will not be visible to smartthings.
There is some custom code that was developed by a community member that does allow an accessory device like the hue motion sensor attached to the hue bridge to also show up in smartthings, but there are variable reports as to whether people find that reliable.
If you do have it connected to the hue bridge and you have hue bulbs that turn on because of the motion sensor, then you could set it up so that smartthings sees that Bulb come on and then does something. That will work for some use cases, but not others.
So it really comes down to the details of what you were trying to accomplish.