The switch uses the table to know where to send packets of data. Every computer has a unique MAC address and the switch will record what port each MAC address is plugged into. It really doesn't effect performance enough to notice it. However, if it can hold more address than it has ports (an 8 port switch that holds 100 or whatever addresses) is because it can track a MAC address that may be hooked to another switch. This way it knows to send the data out port 8 (or whatever port the other switch is in) and it helps to speed traffic.