It's extra and perhaps nice to know, but it's not critical. The copier, which is in the hall, is broken. In other words, there is only one copier. Because they do enclose nonessential information, and because most humans are curious and will focus on the unexpected, they're apt to read what's in the parentheses first. Stick with commas, which are unobtrusive.
Essential material is necessary for complete understanding.
This information is considered essential because it specifies which copier you are talking about. The red mittens that are on the table are Sam's. This says that of all the red mittens you might find around the house, those on the table are Sam's. Therefore, the information is essential. The red mittens, which are on the table, are Sam's.