You don't have to use anything.......but I believe there's a benefit.......many of the powders have an anti fungal agent, but I had a guy say foot powder accomplishes the same thing.......and I've heard people say they don't do anything at all without any difference in success rates.
I do use them...maybe out of habit... I use a gel for my aero cloner more to seal the cut against an air bubble than anything and I use a powder if I root in rw...... I think I get more roots with it than without....... I also thought clonex really increased the amount of roots as the whole stem seemed to burst with roots as opposed to a few nodes here and there.....but when they jacked the price too high, I quit using it. As long as you can get it to root, so it can feed itself....it's all good.
A package of rooting powder doesn't cost very much and lasts quite a while, but there's nothing that says you can't get by without it. For pennies a plant, I'll continue to use it....... They're all basically the same thing....active ingredient Indole Butryl whatever...or something like that.....and there are claims that it's a carcenogen(sp?)..