If I've already adjusted my images in Lightroom and it's set to 72 ppi, what’s the purpose of the Output Sharpening settings?
Output sharpening is designed to compensate for loss of sharpness caused by resizing and/or printing.
You should first do capture sharpening in the Develop module, so that the image looks reasonably sharp at 1:1 magnification.
Then select appropriate output sharpening when exporting. In most cases, low or medium sharpening should be suitable.
Note that there is different output sharpening for screen and print.
Also note that the ppi number only affects printing. If the image is intended for screen viewing, the ppi is irrelevant, only pixel dimensions matter.
Thanks Bernsten. Out of curiosity, what happens if you don't select this option?
If you don't select that option then there will be no additional sharpening applied during the export. If you don't need it or want it, don't use it.