They want to force you to interact with them. It gives them the chance to sell you even if the property isn't what you want--they'll always offer to help you find another property, or they "have another listing that'll be just perfect" for you. Its a sales tactic.
Or the OP is looking at unimproved land/lots which don't have addresses. I'm currently looking at properties for a future vacation home and at least here the electric company assigns addresses--so if the lot doesn't have electrical service it won't have a real street address. You're lucky if the realtor lists the tax ID so you can look it up in the county database.
It is the first reason. It is Germany. By law, the realtor is required to be paid, if you got first contact through them. With that they want you to require to sign the required paperwork, that they work for you.
But as another poster pointed out, it costs everybody time when you need to make an appointment and then at first sight you see that this will not work out.
Or the OP is looking at unimproved land/lots which don't have addresses. I'm currently looking at properties for a future vacation home and at least here the electric company assigns addresses--so if the lot doesn't have electrical service it won't have a real street address. You're lucky if the realtor lists the tax ID so you can look it up in the county database.