To the specific benefits, XLD attempts to correct all errors encountered using the appropriately named and highly regarded CDParanoia engine. XLD also informs you of any errors that could NOT be corrected, whereas iTunes does not. With iTunes you find out later, when listening.
Secondly, there is the capability to compare your rips to the Accurate Rip database for checksum comparison between other known rips. This is often considered the primary benefit of using XLD, or other rippers with this capability, as this CAN be the ultimate in 'peace of mind', or the source of loss thereof.
Additionally, XLD will correct for any offset in the drive itself. The offset correction is automatic, in that XLD defaults to the known settings for most common drives, and you need do nothing. These is (admittedly very) little evidence that offset can affect the sound, despite that it has been reported anecdotally.
You can also set up XLD for something called C2 error correction, drive permitting.
Finally, XLD offers many options in it's ripping function, seemingly more than anyone could need. For example, the list of available formats to rip to includes formats I didn't know existed. Ditto for options like re-trying with bad disks, scanning disks for potentially bad sectors, etc.
With pristine discs, iTunes probably (some would say definitely, and Kent Poom famously said definitively') rips as well as other programs, but gives NO indication of any undesirable behavior with discs that are less than perfect, and thus the aforementioned lack of 'peace of mind'.
The developer for XLD is a very dedicated Japanese fellow, who is constantly improving and/or adding features to XLD. There was a release Christmas Day I think.
So, in the end you trade one 'peace of mind' for another. With iTunes, you lose peace of mind due to not knowing if your rips are error-free. With XLD (and other programs), you lose 'peace of mind' due to now knowing if you've set all the available options correctly, or when your rip's checksum does not match the Accurate Rip database, or perhaps even due to wondering whether to re-rip now that a new feature offering even more 'peace of mind' is available.