There are a number of things you can do with an express card such as buying an adapter that will function as a multi-card reader (sd cards, cf cards, etc). Other options (where compatible) are sound cards, USB ports, even hard drives I believe (ssd drive express card slot). You can use it for pci expansion, wifi, tv tuners, serial ports, and I am sure a number of other things.
Will you miss it? No. Why not? Because if you have to ask what it is for and what would it do for you then you probably won't miss it at all.