There are several reasons for including a map card with your wedding invitations.
Inviting out-of-town guests who aren't familiar with the local area surrounding your wedding site.
Planning a destination wedding at an out-of-town location.
Having your wedding reception at a site that is a distance from the ceremony locale.
You have a few choices in that you can simply have cards printed with driving directions, you can include a map on the other side of the directions card or you can have just a map card printed. It will be up to you to provide your printer with the specific directions and either a hand-drawn or computer-created map if that will be also be included.
These cards are inserted with your wedding invitations just as a reception card might be and are typically printed on white or ecru paper stock unless your invitation style happens to offer a matching map card.
Including direction and/or map cards with your wedding invitations can eliminate frustration for your guests by providing them with detailed and concise directions to your ceremony, reception or both if they are unfamiliar with the area.