Because it’s not really required and is a bit dated. You remember the early days of the web when people used to read out all the “http://www.” stuff a character at a time ? The “brand” singletrackworld now requires less guff to be entered in a browser to access the content.
Except, you can omit the http bit and a modern browser will work it out, but the www portion of the URL is part of the address and is reliant on the server end responding sensibly. (A browser can readily guess, but it shouldn’t have to.)
In most URLs, the first ‘word’ is the name of the website. Conventionally this is “www” but there’s no real reason for this other than habit. You could call it “web” or “forums” or “Brian” for the difference it makes. Historically these would probably have been separate physical web servers, but a modern web server will typically host any number of sites and look at the requested name to serve up the correct one.
So, STW’s web server appears to be configured to respond to two hostnames; “www.” and a blank hostname. The www. address looks to then redirect to the blank site. Why? Who knows, “why not” may well be the answer. As ATP says, it’s not really needed unless you’re hosting multiple sites under one domain (eg, if STW wanted to have http://www.singletrackworld.com, forums.singletrackworld.com, gritcx.singletrackworld.com etc.)