An veteran manager (many years of experience) told me

in his experience, rewarding an employee with a cash bonus or small raise, had only a very short term effect on the work of the employee. Much more enduring effects came from public recognition and small surprise awards. Annual rewards meetings giving gifts (such as home electronics) to top performers, surprise gifts of “Dinner for two” checks of, say, $200, etc. It was the public recognition that had the longer effect, not the size of the award.

Yes, its because, man as Tom Peters describes, is a “sucker of praise”.  Its this weakness HR folks tap into very well.