Sometimes the caps fit inside the fitting. That could be the case there, but in any case, yes, of course that is a display piece. They do not show you a picture of the part that they are about to ship to you.

You don't need much of a seal for this application. The pressure is about zero. The idea is to keep the air out, so as to keep out moisture.

If you have a dryer that has been sitting with no caps, do not use it. It is spent, and won't do the job.

The more time spent between installation of the dryer and evacuation, the more water the desiccant will absorb out of the air in the system. And, as I said before, you have no indication that the system is wet until you get leakage resulting from the acid formed in the system.

However, I will agree that you probably won't get much water in the dryer in most cases. It does come full of nitrogen and it's not like it's under a vacuum when you connect it to the lines, so it won't suck the air in from the system. But temperature changes (e.g., driving the car) will move the air around somewhat, and you have no idea how much.