Where a scale is only used to perform simple weighing very few keys are required, such as a TARE key (left and right tare keys are better), perhaps a PRINT key for recording results, maybe a UNITS key for changing the unit of weight, and a POWER ON/OFF key.  It works for a single purpose unit with no security features.  However, today’s applications almost invariably require more data output from the scale.  Things such as; operator ID, project ID, Scale ID, date and time stamp along with result, and all from the scale.

Many scales are provided with 6 to 8 keys where several of them are multiplexed to provide extra functions (A and B pressed simultaneously = C), or a key may be used to represent numbers by the number of times it is depressed, and another key used to advance to the next digit.  The display is used to inform the operator of keystroke operation and meaning.
The better scales provide more complete keyboards that include things like; 5 navigation keys (4 direction keys and 1 ENTER key), 10 FUNCTION keys that also serve as numeric entry keys, 2 TARE keys, and, perhaps, a CLEAR key.  However, even with all that, an alpha-numeric keyboard is still best where large amounts of external data must be entered.  This is most often a laptop computer keyboard accessed thru an RS-232 port.

Almost all modern keyboards are sealed, and both waterproof and dust proof.  They vary considerably in tactile feel and feedback, which can be frustrating for the User.  Keeping the keyboard clean requires care as the legends may be printed on the top surface without a protective layer.  If a key becomes intermittent it usually requires that the entire keyboard be replaced, which can cost more than the scale did originally.

In the end, it is the User who must decide the value of the keyboard, for it is the User who must pay the price, either in the original purchase of the scale or in the inconvenience of using it.