You can put things in it like heaters, filters, protein skimmers ..so that they aren't all hanging all over the display and making it look ugly.
You can put filtration in it that you couldn't to the display, like in salt water for instance. It's very popular to put algae in the sump so it absorbs nutrients, This algae would not look good in the display.
And there's also the extra water volume. The more water you have, the easier it is to maintain good parameters and the longer it will take to spoil it.