This image courtesy of Icebreaker.com perfectly illustrates what baselayers are! But they're far more important when doing something active such as riding motorcycles. And believe me, they'll vastly improve the feel and performance of your existing gear.
Awhile back, I shared this link to REI's website, which breaks down the various kinds of materials, what they do, when you need it and how it works.
Aside from the technical reasons why baselayers work well, the bottom line is that you need something between the surface of your skin and the lining of your riding gear to keep the moisture at bay, (not eliminate sweat completely, but keep it under control) and increase your overall comfort.
Dainese D-Core Women's Shirt
It's 85 degrees out. You're wearing a cotton tshirt and your arms start go get all sweaty. Then the lining of your jacket starts to stick to you and all you can do is think about how hot and uncomfortable you are. You can't get any air in between your skin and the lining because it's sitting right on top of your skin.
Take a look at this illustration and read this awesome article by Fix.com about how this works. When you wear the right baselayer, you will be far more comfortable underneath your gear. Yes you are still sweating, but this keeps you more comfortable.
Photo and Article Courtesy of Fix.com
Imagine a lightweight, synthetic layer that sits but doesn't stick to your skin. You sweat profusely but the material sucks up the sweat as much as it can, allowing the air to actually flow over your arms. Now imagine pouring a little cold water down your arm (say you have a vent on the bicep) and then you hit highway speeds. Thewater actually feels cool and better yet, the shirt is absorbing the water cooling your skin a little bit. There are a ton of options out there, synthetic, bamboo, merino, something for everyone.
You don't have to spend a lot of money, either. Try going to your local sporting good store and try inexpensive running baselayers. You can easily find a shirt and pants for ~$25 each. Just make sure they have flat seams. Raised seams will drive you crazy! And yes, bottoms are just as important as tops. Moreso if you wear leather. It will also make taking your gear on and off a breeze, no matter how much you're sweating.
Anything you can do to maximize comfort will minimize distractions. If all you can think about is how hot you are, how can you think about that car 2 lanes over about to merge erratically into your lane?
