Why does a recipe require erythritol and some other non-sugar liquid sweetener?

For example this low carb brownie recipe needs erythritol and stevia or something like liquid Splenda. What does erythritol do that stevia cannot? If I choose to only use stevia will my brownies be ruined? Somebody help, please. I'm about to enter phase 3 of HCG. I need low carb stuff but for some reason I don't think that erythritol is even allowed really because it is a "sugar alcohol". It's calorie-free but I am very skeptical of things that say sugar. I don't want digress in my weight-loss during the 3 week recover period. We are supposed to keep the carb intake very low during this phase.