For me it’s cost, convenience (not having to remember to buy something regularly), less waste - but I also use hypochlorous for other household disinfecting so making large quantities is useful to me. If you’re just doing skincare with it, I can see it making sense to buy it premade, especially at first when you don’t know how much you’ll like it. Though I will note that, while the information can be intimidating at first, once you make it a couple times it’s extremely simple going forward. Now that I know the ratios of vinegar and salt I need for my own machine, I just dump things in and run it and it’s good every time without having to make any adjustments.