Loft angle on the putter mostly effects distance control and a little in direction control. The normal loft of a putter is 4 degrees. Most people think that there is no loft on a putter but this is incorrect as loft is necessary to roll the ball consistently on the greens. In the by gone days when the greens were super slow the loft on most putters were 8 to 12 degrees but with todays quicker speed greens the ideal loft is somewhere around 4 degrees.
Why is loft necessary? When the ball comes to rest it settles down in the grass and when you putt you do not want to drive the ball into the ground but lift it slightly on the top of the surface to get it rolling properly. If you have improper loft the ball hits into the ground and will bounce causing inconsistent speed control. Many of you have experienced the ball bouncing and never reaching the hole. The loft being too low or too high can cause bouncing of the ball instead of the initial lift, skid and then roll. If you chip a ball with a low lofted iron you will see it lift, bounce a lot, and begin to roll. Ideally we would want about 20 percent of the distance to lift and skid from the correct strike of the loft and the remaining 80 percent the proper rolling action.
It is important to realize that the same lofted putter used by different players will get different results. This is because every players stroke is different. One player may have the hands ahead of the ball thereby decreasing loft whereas another player may have the hands behind at impact thereby adding loft.
In putter fitting I observe how the player positions the hands in relation to the putter head and observe if that position is maintained at impact. I static measure the loft of the putter in the relation of how the players sets up and addresses the putt using golf fitting loft and lie gauges. Although my eyesight is pretty good I have a couple of tools that help with this. One being high speed video and the other a golf swing analyzer for the putter (GSA Putt).
Putters with bend in the shafts provide offset are often installed incorrectly which can alter the putter's loft.
A good test for this is to hold the putter firmly in the hand and place on a hard surface. Make the face square to the target line and the shaft vertical to the line of the putt. Now let your hands go and if the putter face is square it is installed correctly, if it goes to the right the face is open and there will be less loft that required is it is left or closed it will have more loft than required. Again incorrect loft at impact will cause the ball to bounce and roll inconsistently.
Wouldn't it be nice to know that when you putt all those missed putts that you pulled, pushed or hit off center, is it the putter or is it my stroke? The proper putter loft is just one on the equations in putter fitting and the answer should be it was me and not the putter.