are you getting a new desktop or another laptop? adding a monitor to a laptop can be very helpful if you have a smaller laptop. adding two monitors to a laptop would be a pain unless you have some sort of docking station.

generally two monitors makes it easier to control the size of the individual windows, from accidentally closing the wrong window, not paying attention which window is active then accidentally deleting or adding characters to some spreadsheet field and more. I have been using two monitors for years. having that second monitor saved my bacon more than once.