Your DE has a bar that does the stretching for you. With a straight razor, you don't have that bar to stretch the skin and if you don't keep that skin tight, it'll bunch up in front of the blade if you're not careful. Bunched up skin in front of the blade increases the likelihood that you cut yourself. You don't HAVE to stretch the skin, but ask yourself how many times you want to cut yourself before you just give in and do the stretching