Excuse me some of the reasons that I give here maybe wrong but it is the information that has been given to me over the years for using PDF/X rather than just PDF1.5:
Increased meta data can be included
It's more bullet proof way of transporting files which also leads to smaller file size
Less need for pre-flighting as the Pro version Acro will show areas of problems
A standard desktop export file conversion tool that does not require tools like Jaws or Cute PDF
That files should not have device dependant tags on them (no files should have an RGB tag on them)
That files produced in PDF were totally print ready (nearly the same as above)
To allow better discussion
That PDF/X-3 can allow embedded ICC profile in it
That PDF/X is the baseline PDF and all printers should work from that.
PDF/X allows the use of layers

As I stated at the top this is just office conversion and the list is just made up from what I have digested from info given to me.