The short horizontal dash in a calculator can mean two different things. In 8 - 3, it asks for subtraction. In -3, it belongs to the number itself. A calculator that treats those cases as identical would have trouble knowing whether it is waiting for a second operand or reading a signed first operand.
That is why many scientific calculators provide a dedicated change-sign key, often written as (-) or +/-. It is a unary operation: one value goes in, the sign flips, and the value remains the same size.
Subtraction needs two values
The minus operator sits between a left value and a right value. It creates a new result by taking the second from the first. In a parser, it is a binary operator with two inputs.
A negative sign can appear before a number, inside parentheses, or as part of an exponent. It does not wait for a second value in the same way. Casio manuals distinguish the minus sign used for a negative value from ordinary arithmetic entry on several scientific models.
A negative exponent is not a subtraction request
Scientific notation makes the distinction easy to miss. In 4.2 x 10^-6, the negative sign belongs to the exponent. It says the scale moves left; it does not ask the calculator to subtract 6 from 10.
The separate key protects the expression structure
Human readers use context instantly. A small calculator parser has to encode that context in key actions. Keeping subtraction and sign change separate reduces ambiguity around powers, products, parentheses, and the start of an expression.
Minus is a relationship between values. Negative is a property of one value.