As stated above, LR does not alter your original images. Rather it saves a list of changes you made to your image in the catalog. You can see this list in the History Panel of the Develop module. In other words this list is a recipe of changes to make to the original file to give you the resulting look of the changes you have made. That's all you really need if everything goes well.
However, saving changes to XMP offers two benefits. One, as mentioned, is the ability to open the image in something other than LR and if that program is designed to look for the XMP data it will show you your edited version (to the extent that the non LR SW interprets things the same way as LR intended). Along with Clee, I consider this a risky workflow and would recommend against it.
A 2nd benefit of XMP files is if something terrible happens to your catalog rendering it unusable. Yes, this does happen.
Sometimes through operator error, sometimes through SW or HW failure, but it does happen. This is why it is extremely important to have twice as many backups of your catalog as you think you'll ever need. But, even so disasters happen. Talk to folks in the path of a hurricane or tornado. Talk to folks in Paradise California. Many such folks thought they had backups up the kazoo until they discovered that all the backups met the same fate at their house. One of my clients even remembered to grab the external disk containing all his images as he ran out the door, but without the catalog and without XMP those images are the originals as they came from the camera. 10 years of work up in a cloud of smoke. Had he saved XMP files, once he got his new computer, plugged in the saved EHD, and imported the images into a fresh new catalog LR would show him the images as he last saw them (not the steps that got him there, but the final position of the sliders and targeted adjustments).
In project management, risk mitigation is a major component. In a nut shell there are 3 things to consider. 1) Likelyhood of the bad thing happening, 2) effect on the project if it does, and 3) cost to either prevent the event or to mitigate the effect. So, for me, saving the XMP's is "little cost" in time and disk space. The likelihood of me having no usable catalog quite low as I have both local and cloud based backups of my catalog and images). And, 3rd should such a disaster hit (e.g no usable catalog) it would be catastrophic. So, I auto save XMP for the same reason I have an insurance policy on my house and car.