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When making chocolate unsaturated fat is used for a good melting point (and perhaps health reasons.)
To make chocolate edible it has to be a bit more solid, so other fats were required. Therefore they use trans fats, which are unsaturated fats made saturated in cis or trans form.
Why do they do that instead of just mix it with normal saturated fat? Why use trans fats?
Catalytic hydrogenation tends to make trans fats... unfortunately catalytic hydrogenation is the easiest (read lowest cost) way of hydrogenating fats so it became the industry preferred way of hardening fats before anyone realized there was a health risk to trans fats. Cheap plant oils can be hardened by hydrogenation... and hydrogenated plant oils are generally cheaper than naturally saturated fats.
Because they are unnatural, trans fats also don't go rancid so quickly... so goods made with trans fats can have longer shelf lives. Which cuts costs. Given so much of mass food production is severely cost conscious, it was inevitable that trans fats became widely used and are now having to be removed as ingredients.
Consider olive oil spread. Olive oil is composed largely of unsaturated fats which do not "stack" easily onto each other at the molecular level, and thus are a liquid at room temperature. If you want to use it as a spread or in baking it is usually needed to be a solid. This is done in a process called Hydrogenation where the unsaturated molecules become saturated with hydrogen, thus as you stated making trans fat. This process is conducted in order to make often healthier vegetable oils (usually unsaturated) into solids at room temperature.
