All else being equal, developing constent contact with the putter face is the most important aspect of putting to master. Why? Without consistent contact you can never truly contol your aim OR your speed.
Any deviation from the sweetspot of the putter face will affect both the distance and direction the ball travels. Hit the ball a little on the toe and your putt will move right and go a shorter distance than if you hit the center of percussion on the putter.
A great drill to do with these it check your contact at different distances from the hole. How is your contact on 25' putts vs. 10' putts?
While the decals are not reuseable they can be used for many repetitions until you have established an impact pattern. Using a putitng impact decal will remind you to focus on this important essential and check your performace.