John,

A pack shrinks the size of the file. However, if you subscribe to the idea that any particular table will have a size that it will grow to and then for the life of the application it will float around that size losing a few record to deletion and then gaining a few through addition, then shrinking and growing the file size will introduce disk fragmentation to the file slowing access time.

By recycling the deleted records the file will grow until it has that magic number of records, then it will no longer grow anymore. A defrag of the disk will put the file contiguous. The recycling of the deleted records will maintain the contiguous nature of the file and prevent further fragmentation from being introduced.