A truck simulator in this case, is more 'complete' with real trucks instead of made-up trucks like ETS1 (wich were based on real trucks, but they still were different). With those licenses, they are allowed to use the names, and truck-designs to every inch of the truck in their game(s). Just adds a little 'extra' to the realism-feeling.
Just like you can buy several cars in Need for Speed Pro Street. You can say to your friends "Ohh yeahh, I have just bought the BMW Z4M Coupé, and I'm now driving it at the Nurburgring!", you could now say to friends "Ohyeah, I've got the newest DAF XF95".
No idea, I think, that if a developer decides to base a truck on a real truck, that it can't be close unless he has a contract / license with the manufacturer of the truck.
From those manufacturers eyes, it would mean, giving certain amount of people the 'sensation' to drive in their truck, instead of a made-up one. Its also a bit of advertising.
Well I believe ETS1 already had quite large fanbase, and even without realistic trucks it was quite huge success.
And the trucks on ETS2 are fantastic, and I wonder if they just need to notify every single small detail error they find on the 3D model before they can accept the licence.
Looks like its going to be awesome. I will be buying it whether there are the real truck logos or not. If you remember in ETS you could download a mod anyway, to give all the trucks their real logos.