The dumbbell floor press is a progression I use with individuals before we look to advance them to flat bench chest press and bench press. The lying position means that the range of movement is controlled with minimal stress to the shoulder joint. This makes it a perfect exercise as a progression on from incline chest press as well as press ups. Often in individuals with unsuitable shoulder stability the scapula cannot hold position if you go to flat bench or chest press too early in their progression.
Twin this with poor torso stability and you ultimately leave yourself open to more shoulder issues. The floor press enables you to have a controlled range of movement while maintaining a suitable elbow position. Often in the pursuit of more weight the shoulders will flair out making the movement more nech dominant and placing more stress on the acriomioclavicular joint.
As you can see in the image there are a lot of ligaments that enhance stability at this joint which helps keep movement stable. From a muscular standpoint the movements you undertake will determine the pressure placed upon these ligaments. This can be shown in the image below- as joint position needs to be maintained as you lift the arm to vertical the more stress placed upon the shoulder joint which can lead to irritation and injury of the joint capsule as well as the structures of the shoulder used for shock absorption such as the bursa and cartilage.
All in all the floor press is a good compromise and a starting point for progressive loading for those with stable shoulders and can be a useful inclusion for newbies and advanced trainers alike.