With this grip you can really feel the rod and apply power to it by stiffening and relaxing your thumb. For beginners this is very important and the only grip that I would recommend.
Without the thumb on top, it will leave your rod grip just like you were holding a baseball bat. You would have very little control in making your rod tip track correctly. It is extremely easy to let your wrist break over on the back cast, a big no no when casting. And in order to apply power to the rod, your pinkie finger would be doing a lot of the work. Most likely, your weakest digit.
Another way to grip your rod is with your index finger on top.
For me, at least, it is harder to apply as much power to the rod and also harder to control higher line weight rods and/or just heavier rods. But, using this grip with lighter line weight rods, I can deliver a softer presentation. Generally I will use the index grip with med. action 4 wt. rods and lighter. A fast action 4 wt. might work too, but I don’t own one. I can really “feel” the rod this way with med –med/slow actions and almost exclusively cast my 1 wt. with the index grip.
On multi day trips, my thumb and hand can get pretty worn out casting fast action rods. But switching rods and my grip to the index grip can keep me on the water longer and more comfortable which, of course, means more caught fish….I wish !! There have been several times that I would like to switch grips when casting an 8 wt. with a bass taper line, but for me to do that would end in an emergency room visit with a broken index finger. I know guys that can do it with 9 wts., but not me. So, until I can learn to cast left handed, I’ll just have to do the best that I can.
Try switching between these two grip types, using different rods and see if it helps or hurts. You might find, like I did, that you prefer the index grip more for certain rods or actions than the thumb grip.