Starting with a single ad means not knowing why when it does not work. More than one variant lets you choose which message works by looking at the data.

In 14ads

A campaign produces 2 variants by default; you choose the number during production (up to your plan's limit). Each variant is a separate test of script, hook and voiceover.
The optimisation loop recommends scaling the winning variant and pausing the one that performs poorly. The default mode is recommendation: the user approves any change that affects the budget.

How the engine uses this metric from ad production to publishing is on the how it works page; credits and plans are on the pricing page.