They arrive before you do and park where the car can wait without becoming a problem. They come to you rather than making you find them. They take the bag out of your hand at the door and put it in the vehicle themselves. They hold the rear door, and they close it behind you without slamming it.
Inside, the temperature is already reasonable and the route is already decided. If you want to talk they will; if you open a laptop they will not. The car is quiet enough to take a call in, and clean enough that you can put papers on the seat next to you without thinking about it.
At the other end they get out first. On a return trip they know the pickup point is the same door you came out of last time, because the same chauffeur is often assigned to the same passenger across a day or a visit.
None of this is remarkable individually. Together it is the difference between transport and a chauffeur, and it is why the service is priced as a person rather than a distance.