For a shallower DOF.
Because my flashes aren't Canons I don't have HSS... so I am limited to 1/250 or slower shutter speed. This would normally give me an aperture of f5.6 or 6.3. The ND filter allows me to go wider.
I was after a specific look that I didn't achieve. I ran out of time for tonight.
My biggest beefs are that
1- I focused on the eye on the dark side of the face
2- I slightly over exposed... Her muzzle is too white IMO
3- I should have gelled the flashes
Live and learn, I'll fix them next time