So he can browse without people seeing what it is. If you use a browser that gives you tabs, then it shows a pot leaf and when you're not in any specific thread it says "Marijuana Growing". That means that you've got to switch desktops (not likely for most users/ability) or close out the browser so you don't get caught. This site could not normally be called "safe for work", for sure.
For instance, my "office" is in our living room, and anyone who comes in through the front door has to walk past my desk, where they can see exactly what I'm doing if I'm on the computer. Now, I have pretty much no worries, plus I just use my mouse scrolling button and I can switch to a blank desktop. But others are not so fortunate.