File systems and the reason for block allocation??

I'm taking a class on operating systems and one of the chapters covers File Systems. During the reading it goes on to explain that the file system uses the method called block allocation to keep track of where specific files are stored. It does this by creating clusters which correlate to sectors, heads, and tracks on the disk. It marks the file at cluster 346 insted of the absolute address; track 15, sector 56, head 1. My question is why. Why does it assign clusters insted of using the absolute address when it has to look up the absolute address of the cluster anyway? Why not cut out the middle man and just remember the location by the absolute address? Also where can I get information explaining this in detail? I have to write a one page report on the matter.

Thanks,
midgaar