Deep balls that bounce low give you more time to make a decent swing at it, deep balls that bounce high means you have less time and end up hitting shoulder height.
To experience this you would need to generally position yourself on the baseline rather than behind or simply have a deep hitting partner... if none of those apply to you then maybe that could be why you are struggling to understand this.
Okay .... let's see if I can parrot this back to you ....
First we were talking about grass. Now, it has been my experience that a deep ball that bounces low on grass gives very little time for the player to reach the ball before it bounces twice and the point is lost. Or it bounces low and skids out if it has been hit with any kind of serious pace. Also, I am not in the Agassi class i.e. have the ability to stand on the baseline and half volley. Are we getting any closer on this or am I still not getting it?