Because it's easier then buying one
When connecting two computes together (e.g. your laptop to a server or router) to get access to a console you need to use a crosslinked RS-232 cable, usually with two female DE-9 connectors these days. This cable is more commonly known as a null-modem cable for historical reasons.
Of course you can easily buy a cable under the name of "null modem cable", but you're almost guaranteed to get a sloppy made one. In fact I haven't been able to find a decently made one. The problem is that according to the standard the DTR is supposed to be connected to the DSR and CD, but most cables as sold won't connect the carrier detect line. No idea why, perhaps making the bridge is something other connectors don't need and therefore expensive to automate for production?
So to make one yourself you can cut up an existing cable and solder new DE-9 connectors onto it yourself, keeping to the proper schema. It's really not that hard, all you need is a soldering iron and two D-sub 9 pin connectors and corresponding hoods. I'm guessing you could even use cheap CAT-5e networking cable instead of cutting up an expensive pre-made null-modem cable since Sun uses network cables for their serial cables (whith an adaptor).
Now most devices actually seem to ignore the carrier detect line anyway. But in case you are lucky enough to be using IBM's AIX or IVM on one of their System p5 servers you will discover that you are able to log into the service processor with a "normal" (i.e. non-standard) serial cable just fine. You will also be able to perform an installation from that searial cable just fine. But when you then need to access the console of that just installed system you're stuck and just see nothing. Until you use a proper cable that is. (Just for the record, you can most likely telnet or ssh into the system by now, just try it).
And if my love for AIX is not yet clear from the above paragraph let me be a bit more explicit: if you are in a position to choose get something else then AIX. Sun's hardware as well as Solaris are lovely (they ship with a serial cable for instance - as does Cisco). Obviously GNU/Linux is a great choice for an OS too. This doesn't mean that I approve of cable vendors not making RS-232 crosslink cables propperly of course.