Are you organising open days? Then you want your organisation to leave a good impression. You can let interesed people register for their visit. In this way, you know how many people you can expect. Maybe you want to give a presentation or arrange some workshops. Interested people can register themselves in advance for these sessions through the register form.
Also onsite people can be registered. In this way, after the event you can send your visitors a mailing to thank them or an offer.