Originally Posted by AlgarveCycling

I absolutely appreciate that cycling in jeans or cargo shorts is perfectly fine in most cases but those who prefer lycra often ride with much greater intensity, than those who don general use clothing...the abrasive movement between saddle and upper thigh is harsher. Chafing is less of an issue if you aren't cycling with any great urgency. Even for those who wear lycra and have very good chamois, often a good chamois cream is required in addition on longer races or endurance events where time is a factor in the performance. You are, essentially, protecting yourself from your bike which causes the chafing when it is ridden this way.

Now, you can get cargo-type shorts with chamois inserts too - you don't need lycra. For my first foray into XC MTB I wore cargo-style shorts like the downhill riders tend to do and then added a chamois insert because it was more comfortable for me to do so. I have since adopted lycra because I now race MTB and the other benefits of lycra come into play.

I commute, go for fun rides etc in jeans, regular shorts etc. But for training and racing? Lycra. The reasons are many, the chafing issue above, extra padding for comfort over rougher road surfaces, the stretch of the fabric which is more comfortable and accommodating of a more aero racing position, the more efficient handling of sweat - the lycra allows your sweat to move through the material, known as 'wicking' so that it can evaporate, cargo shorts and jeans are not nearly as efficient - and finally, of zero importance to the casual cyclist or non-performance orientated rider, more streamlined, aerodynamic clothing reduces drag and ergo improves performance where speed is desired.