Just to be clear, ALL seats on ALL trains between Baltimore and Philadelphia require a "reservation." However a "reservation" can be obtained at the ticket counter at any point before the train's departure. If money is no object, then the only particular reason to get an advance reservation is to ensure a seat on the specific train that you want (trains rarely sell out except over holiday peaks or during weekday "rush hours"). But as others have noted, buying a ticket in advance is almost always cheaper.