An inset playing area has come to define board game tables. Honestly we were a little skeptical, until we played on one. Playing a board game on a table is different than eating a meal, and the inset playing surface accommodates those differences.
While playing a game, it is natural to rest your arms on the table. The rail provides the perfect place for this. The lower playing surface gives you a bit of a bird's eye view. With your opponents' arms up and out of the way, their player board and tableau is more visible.
The best part is that dice, wood cubes, and little chits seem to fall off the table a lot less when there is a rail around the edges.