Why letters

Describing every case at once.

A general statement

Writing area = length times width says something true for every rectangle, not just the one in front of you. That generality is the whole point of algebra.

A variable is an unknown value; a parameter is one you supply.
An expression has no equals sign; an equation claims two expressions are equal.

Reading before solving

Ask what each letter stands for and in what units. Most word problems become easy once the quantities are named honestly.