I have never used a carabiner on my setup and see no need to.
I use a loop I sewed on one end of my webbing to act as a channel for itself once it has been wrapped around the tree. This can also be done by tying a figure of eight loop in your webbing.
Lots of people seem to use a carabiner instead of the loop or a dutch clip instead of the loop.
I see no obvious advantage for this. It's heavier and bulkier. Is it because of the breaking strength and the loop becoming the weakest link in your setup? I use 500 kg breaking strength 1" webbing.
So why do people use carabiners?? I'd like to be informed about this.
If I need it at all, I use soft shackles (since I got myself a loooot of amsteel )