The "thumb ridges" seem like a feature that appears on the knives that I have that are considered skinning knives. Gerber seems to have particularly used this feature. (If you have an older Gerber from back when they were of somewhar better quality then you are fortunate.) I assume that it's to provide a surface with some grip to place the thumb on when you have a bit of slop (blood, fat...) to contend with while you are dressing a carcass.

In culinary, placing the thumb or the index finger on the back of the blade one of the grips that they teach you...very functional for different types of work.

Best

AA