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.
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.