The key itself is dumb; the server is smart. In the Serial Key Unlock World, the key is merely a token. The real magic happens when your computer talks to the mothership (Microsoft, Adobe, Valve) to see if that key has been used 0, 1, or 1,000 times.
Some professional software (Avid, certain medical imaging tools) uses dongles—physical USB keys. You cannot crack a dongle without hardware engineering. The unlock world struggles here.
The truth lies in the middle. While Adobe won't go bankrupt because you pirated Photoshop, the cumulative effect of millions of unlocks destroys the market for mid-tier software developers.
: They prove that you have legally obtained a copy of the software.