Memopak 16K gives you a massive 16K of Directly Addressable RAM, which is neither switched nor paged. With the addition of the Memopak 16K your ZX81's enlarged memory capacity will enable it to execute longer and more sophisticated programs, and to hold an extended database.
The Memopak comes in an attractive custom-designed and engineered case which fits snugly on to the back of the ZX81 giving a firm, wobble-free connection. (A strip of Velcro is supplied just to be sure)