I don't think its necessary at all. But it is nice, especially if your wedding has some unusual elements. For example on ours, we included that the ceremony and reception is outside and on grass, that valet is included, and that we didnt do a room block, but plenty of hotels are nearby.
I wanted to include the grass thing in case it would change how some people would dress. I hardly ever wear stilettos, but we have family that do, so I wanted to give them the heads up.
I don't think it's required, but it's nice to have. This is what's on ours:
DO YOU HAVE A HOTEL ROOM BLOCK?WHEN AND WHERE ARE THE CEREMONY & RECEPTION TAKING PLACE?WHAT TIME SHOULD WE BE THERE?WHEN ARE RSVPS DUE?ARE THERE OTHER WEDDING EVENTS I SHOULD ATTEND?IS THERE A DRESS CODE?WILL PARKING BE AVAILABLE?CAN I BRING MY KIDS?CAN I BRING A DATE?WILL FOOD AND DRINKS BE SERVED?WHERE ARE YOU REGISTERED?ALTITUDE SICKNESS?WHAT IF I HAVE DIETARY RESTRICTIONS?CAN WE TAKE PICTURES?IS THERE SERVICE AT THE VENUE?HOW DO I CONTACT YOU WITH FURTHER QUESTIONS?