I have noticed that people have something called a 'seder plate' on which they place objects. Is there any obligation to do this? The reason I can think of is the custom of pointing during the recitation of the mishnah of Rabban Gamliel: 'matzoh zo...', 'maror zeh..'. But that would be just two items.
This fascinating article explains the origin of the Seder plate. Excerpts:
The Mishnah (Pesachim 10:3) directs that, following the first cup of wine: “They bring [a small traylike table] before him [the Seder leader], he dips ba’chazeret [“with lettuce” for the first dipping] until [or, according to Rashi, “before”] he reaches the parperet ha-pat (“accompaniment of the bread”). They bring [a second table] before him [with] matza, chazeret [“lettuce” for maror], and charoset and two cooked dishes [in remembrance of the Pesach and Chagigah offerings -- today usually a roasted bone and a roasted egg].”
The original Seder had a definite Greco-Roman and Middle Eastern flavor, most notably a Greek style of dining with the participants actually reclining on low couches, pillows, or carpets around a central location (not leaning on an elbow). The various Seder items were placed on several low tables, which were carried in and out of the room at designated points in the ceremony and placed in front of the Seder leader. This practice is maintained today by many Yemenites and other Eastern Jews.
Since the Mishnah prescribed two separate small tables for the Seder items, there emerged a dispute as to which items would be placed on the single Seder plate. There was also a question as to when the Seder plate would be placed on the table. Most rabbis directed the Seder plate be placed on the table after the Kiddush and before the first dipping, while others (Vilna Gaon in Ma’aseh Rav) waited until after the karpas has been eaten, the point when the second table was brought before the Seder leader.
The article contains further customs and info on why the plate became common.
There is no need to have a specific plate set aside as the "seder plate". Many do so because they go out of their way to have a decorative plate in honor of the holiday. The custom I follow does now have a specific plate for this purpose and is not set up prior to starting the seder but the first thing to do is set up a plate when sitting down with the necessary items.