The barbell snatch is complex, and something that's difficult to master even with a coach. Simplifying this movement by eliminating the involvement of the ankle, knee, and hip joint by using dumbbells and taking a seat is great for lifters looking for some size and conditioning.
The best part about them, unlike barbell snatches, is that they make you responsible for your negative reps. That forces you to be much more honest with the weight you choose, especially when doing sets of 6 or 8 reps. Moreover, similar to a Z press, this movement will quickly expose mobility issues since your seated position has no backrest.
If you can't hold the weight directly overhead with an upright torso, it's time to improve your shoulder mobility and the health of your thoracic spine. To say this focuses on just the front, mid, or rear delts would be selling it short because it tackles all three, depending on the phase of the lift.
As a bonus, you'll also light up your traps. In the video, I'm using 35-pound dumbbells, and that's more than enough to do the trick, especially if fatigued from base shoulder movements. Based on their explosive nature, however, keep these closer to the beginning of shoulder workouts.