The difference is just organizational. Usually most unix distributions choose to place in the root directories (/bin, /sbin, /lib, ..) only what they consider the most necessary programs and their associated libraries, that one might use to do some basic work (i.e. for troobleshooting purposes) on the system, in a possible event where the /usr directory is inaccessible.
The /usr directories contains usually the files of more than 95% of the installed applications.
Of course there is no necessity for any directories with specific names to even exist, however it has been established as common practice and many programs contain hard-coded references to standard paths, so it would unecessarily complicate things for a distribution to ignore them. Besides the current structure is convenient in multiuser environments, offering greater administration flexibility.