The Passover seder is a home observance, and was not traditionally done in synagogue. Here in the U.S., congregations and Jewish organizations have offered public seders for generations. They exist to offer a large community experience of Passover to people who might otherwise be alone on the holiday or unable to create their own seder. Some people will participate in a temple seder one night and attend a home seder on another night of Passover. Temples also offer model seders that are usually geared for the Passover beginner and for families with young children who may not yet be ready to create a big holiday experience at home. You can practice at a model seder and then go home and make your own.
Temple seders make good learning experiences for interfaith families, especially when neither partner in the household has led their own seder. These seders are also good for single people who are away from family. They also serve as a comfortable training ground for people who are considering or have just gone through a conversion process. Anonymity in large crowds often can feel more comfortable for learning than feeling put on the spot at a small private gathering. In any case, however, the job of the leaders is to make the guests feel comfortable enough to join in and safe enough to say no to participating if they are uncomfortable.
Temple seders are often shorter than home seders and are also good for people who want the full experience without the time in preparation and the duration of the meal itself.
The Guide to Passover for Interfaith Families is also available in PDF and Word formats.