Apart from building 'trust', calculator plays important job of communication - people in places like Zambia probably do not know how to write in numbers in English (or are not very fluent) and also probably do not know how to say the numbers in English.
Being traders however they would recognize numbers readily, so they communicate via calculator by punching out the numbers. That is, calculator is used even when there is no calculation involved.
[Edit - Corrected the Roman numerals to English numbers]