If you specify a SORTWKdd DD statement, the system allocates the primary work space for it before DFSORT is invoked. DFSORT may or may not use all of the primary and may or may not get secondaries. So you can end up allocating space that isn't used.
If you don't specify SORTWKdd DD statements (or your site's installation default is set to deallocate them), DFSORT will use dynamic allocation of work data sets. DFSORT will adjust the allocated work space to what it thinks is needed. So you can still end up allocating space that isn't used, but the allocation is more flexible.
We recommend the use of dynamic allocation of work space. You can specify up to 255 dynamically allocated work data sets, so very large files can be accommodated.