Almost all universities have an academic rank system. As you already knew, in the United States a tenure-track position starts with assistant professor. An assistant professor has a chance to be promoted to an associated, and then a full professor. Someone said that academic ranks in Europe are different, and a tenure track position starts with a full-time lecturer, and that assistant professor in the US and a full-time lecturer in Europe are equivalent.
Quite often promotion to a higher rank is associated with salary raise. Also, one's promotion may provide a sense of achievement for him or herself. Other than these, what is the meaning of academic rank to you? Does a professor with a higher academic rank have a higher authority than his or her lower counterpart? In other words, does the rank show the hierarchy of authority? Or does it simply mean that the high rank professor deserves a better respect?
Additionally, do you think that universities need to create more OR less academic ranks than the current common ranks? And on what grounds do you think that way?