you need to read some performance and tuning guides.
clustering is interesting when reading (or scanning) more than 1 row via an index.
if you are selecting several rows and they are on the same page or on pages closeby you benefit from these two things :
- usually there are more than 1 row on a page
- pages next to each other can be read in 1 I/O and asynchronously before needing them = prefetching.
clustering helps you avoid I/O's if you put the rows (as much as possible) in the order in which you will mostly process them.