Well, why it costs money - use your brain. Seriously. Why should Dell turn it out for free if not everyone has it so it is not included in the price for the server. It is a part, it costs money to make and store until someone buy sit and Dell is not in the business of giving money away. Seriously, "why do things cost money" questions are not smart.
Now, for the rest.
It normally is a front cover which can be LOCKED (!). This is the most important thing. You install a bezel and you can lock all the hard drives into the server so people can not remove it. Most of the time you also lock access to things like reset button (but obviosuly not the power cord). Whether this makes sense seriously depends on server usage. A server in an open rack - MAYBE (!). Even then the hoster will gladly help you go after other idiot customers playing with your equipment while doing maintenance on theirs. Larger (locked half rack, like me) and even before the real sense starts to disappear pretty fast.
I seriously prefer open servers where I can rip out the discs as needed without playing around with keys for the brezel first.