teceem: Now you're talking about the key as a "DRM mechanism" for (online) support. I agree, that's more effective than what we were talking about here: the serial key as copy protection for the game itself and only that.
But the serial key is least of all a copy protection! It does absolutely nothing to prevent copying; it is used to restrict the *use* of copies.
If a game requires online activation, then without the activation server(s), owning even a thousand keys makes you nothing more then the proud owner of... useless installation files.
Most activation schemes (whether online or offline) ultimately rely on keys and encryption. An online activation server (or the lack of one) doesn't prevent you from playing a game; the game itself prevents you, and the online server enables you, by providing you with the key that makes the game unlock itself. The catch here is that the key you might see printed in your game's manual is not THE KEY; your key is more like the Certificate of Authenticity that is tacked on computers with an OEM copy of Windows. This is what might be confusing you.
THE KEY is often a decryption key that enables you to execute an application or watch a movie or whatever. It might be returned to you (well, to your software) in the process of online activation through a key server, or it might be stored in BIOS (for OEM copies of Windows), in the firmware of your DVD player, whatever.
In all cases, having THE KEY gives you the ultimate say in what you do with your data. The earliest known breach of the DRM on DVDs involved basically nothing more than THE KEYS extracted from the firmware of a DVD player, while better methods can use brute force to obtain THE KEY (see libdvdcss). A common method for pirates to bypass Windows' activation involves inserting (or faking) SLP keys in the BIOS, or by using a leaked VLK. If I try to watch a DRM protected vide on youtube, well it doesn't work in my browser without flash, and youtube-dl won't download the video. But if I had flash, I could find THE KEY in flash's memory or in the network traffic, the key that is used to decrypt the rtmpe stream. And I could use that key with rtmpdump to download a copy of the DRM protected video and simultaneously have the DRM stripped out. More DRM in the movie playback chain involves e.g. HDCP, which, as you might guess, is reliant on encryption.. and keys to decode the encrypted stream.
And SecuROM? Well, the activation process returns you THE KEY. If you have THE KEY, or if you can generate THE KEY... you're in luck.
https://www.youtube.com/watch?v=x6M5bOvv0Fg The old CD key based protection is weak because it effectively places THE KEY (or a code that yields THE KEY offline in software on your PC) in your hands, only trusting you not to share it with the world. So these newer mechanisms either place THE KEY somewhere out of sight (firmware, BIOS, etc.) or they give you a token of authenticity that enables you to obtain THE KEY from an online server.
Barefoot_Monkey: I genuinely don't understand your line of reasoning. Why do you feel that simply involving a key or keys in some way causes you to effectively become the authority on whether you can use the product? Could you please explain what you mean?
I mean you said it yourself: if you have THE KEY, you become the authority. So with CD keys, so with DVD decryption keys, so with Windows SLPs and SecuROM unlock keys and so on.
Of course merely involving keys doesn't mean you always get full access just by having the keys. But most schemes really rely on nothing more than keys! Now if they did rely on something else, e.g. half of your game data being stored on the publisher's server, only accessible by logging in to your account, well in that case keys alone aren't all you need. But people could copy that data and after that it boils down to having the keys.