In the final step of the poll builder, you'll get a familiar view to what you saw in choosing your audience. The difference is that in this final stage, you're collecting additional data points specifically from the people who partake in your poll.
Why should I add demographic questions?
Demographic questions help you further analyze your results by knowing more about each respondent. For example, you may only want to select parents of babies for your poll feedback, but of those parents, you may also want to know if they have kids in other age ranges, or if they own their home or what their income range is and that's when the demographic questions come in.
What if I don't know who my audience is?
This section is perfect for those of you who don't know your target audience yet. Choose a general population and then use the demographic questions to collect more data points that are important for you to know moving forward in your product research. The more segmentation you have to start with, the better picture you'll have to hone in on the right audience that resonates with (and would likely purchase) your product.