The first advantage to labeling special services as primarily for evangelizing visitors is that it allows for special preparation. Prayer bands can be organized to support the ministry. Specific prayer for a specific event is always more desirable and effective than general prayers for a normal service. Hopefully, people will know non-Christians who will be invited. They can pray for people by name that the Holy Spirit will begin working in their lives and prepare them to receive the gospel.
Special preparation can also go into planning the service itself.
Assigning specific services allows one to plan an order of service that is specifically geared toward non-Christians. Perhaps an appealing film, music selection or speaker can be planned so as to entice more people to attend. Secondly, guest services set up that kind of structure that church people can make a concerted effort to invite non-Christians. Often it is difficult to invite a non-Christian to church. The Christian asks himself what there is to appeal to an unbeliever.