Having a single type of accepted money primarily helps consumers in that it makes it easy to compare the price of goods and services.
When there is a standard medium of exchange, such as a nation's currency, consumers can directly compare the cost of items from different sellers. Without this standardization, comparing prices would be more complex, as each good or service might be priced in terms of different goods or services, or different types of money.
While having a single currency can indeed make it possible to make large purchases, this is not its primary function or exclusive benefit, as large transactions can be made through various means of payment or through barter if necessary, albeit less conveniently.
The ease of making a profit on savings accounts is related to financial infrastructure such as banking systems and interest rates, rather than the existence of a single type of money.
The possibility for producers to adjust prices down for sales doesn't depend on the existence of a single type of money. They can adjust prices based on demand and supply dynamics, cost of production, and competitive strategies, regardless of the type of money used.
So, the best response to the question is:
It makes it easy to compare the price of goods and services.