It's all about safety and security.
No such thing as a stupid question. I'm glad you asked.
For me the biggest difference is the security. Brightly lit with people coming and going all night. You have told the management that you will be there, and they told you where to park, meaning they will keep an eye on you. Furthermore, many have extra security all night long, and the local constabulary regularly loop through the lot, even if only to get a coffee. All this helps to keep trouble at arm's length.
Lastly, should anything happen you have somewhere to go or hitting the horn would soon bring management/security to your vehicle.
As Michael said, truckers often do not have a choice. But then trucks are not as easy a target as cars, though I have read about loads been stolen from trucks at rest areas.
Lifey