I've seen many boaters modified their props by grinding a concave on it. Can somebody tell me what is the purpose by doing it? See photos attached.

That is typically called a Barr-Cut and is said to help the prop unload at lower rpm while having little to no effect on high rpm.

So in your opinion, will the barr-cut help to perform better?

It depends on what you are needing as performance increase, but it can help if the prop feels a little "heavy" at acceleration, but seems ok at top speed. I have used this mod on a lot of props with good success, mostly on monos.