Thank you for your helpAlum is an antibacterial and also helps close up little nicks. I lived most of my life never using alum until recently. I like it because where it stings, it shows where the skin was treated a little too harshly. I apply it, clean my gear, then rinse it off before applying a post shave balm. There is no harm in leaving it on too, probably. I try to avoid alcohol, and I am cheap, so I do not use alcohol based aftershave.
Why did I never use alum before? After entering DE shaving world recently, I learned that the crystal deodorant I have been using for years because of very sensitive skin is the exact same compound as alum. When sold as deodorant, it is also cheaper than when sold as a shaving product.
You can get the same benefits from alcohol free witch hazel. It is a skin toner and also has disinfectant properties. I switch between alum and witch hazel, depending on my mood.
I am not sure of the posting outside link rules here, so I will just suggest searching an article on mantic59's Sharpologist website "what is the best aftershave for your skin and your wallet." His info and videos provide a great introduction to de shaving.