The OP was wondering why people suggest other ways of saying things. The reason will vary by the individual, but usually, it is because that person has experienced dissonance. When I encounter a deviation from standard English grammar, I experience something similar to hearing a factual error in the communication.
At that point, I can ignore it, or point it out. Those who point it out may do so to show off, or to be helpful to the person who made the error. They may also be attempting the quixotic task of keeping language usage in general, standardized. In other words, those "errors" in usage could be passed on to the readers who do not recognize the errors.
What I do to minimize dissonance, is to assume the writer is using less formal standards for forum posts than he would use for his resume.