Why preshave

User's choice, at the end of the day. I've recently introduced myself to pre-shave after deciding I was getting too much irritation in places (though that's probably down to use error, as well).

In short, it provides extra protection/slickness from the blade. Some people swear by it, some people will have nothing to do with it. Preparation is where you'll notice a big difference, and technique of course.

Hope this helps! (I swear I'm not following you around the forum...)
Myrsol Emulsion improves any shave I have whether before or after. The late Mr Myrsol even used it for shaving. Applied prior to shaving and left to soak in for a moment or two I feel it assists to prep my skin and adds to razor glide, which is already good when I use Myrsol blue or 1912 soaps. But that's a very personal and subjective viewpoint.

JohnnyO. o/
But what does it do that a decent lather doesn't offer? You will have to be careful in the mix as some oils will clog the razor or make the lather sub par and break down.

Is it a product designed for the sake of marketing and sales?
Oils have been used a lot longer than soaps. They provide moisturizing and skin protection benefits which soaps may or may not sufficiently provide for your skin type (don't know how many times this needs to be repeated). I use DIY oils ,I've never had a problem with razors clogging, or with lathering for that matter.

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I sometimes apply some Noxzema cleansing cream before lathering as that improves slickness. It's the only thing that has made any difference whatsoever, everything else in terms of oils or creams has been pretty pointless.
Yes, I've tried a few pre-shave products and found that they made no discernible difference, other than having to wash the greasiness off your hand(s) before picking up the razor, leaving an oily film on the water and a scum line in the washbasin.