Display advertising is appropriate for all stages in the marketing funnel - to create awareness and interest, drive desire and action. With programmatic buying, you may advertise to a mass or a niche audience, and should expect results accordingly. Generally - measuring effects in the first stages of the marketing funnel is harder, and for the latter stages easier.
An advertiser used to buying visibility in search engines, a natural first step in display advertising could be to retarget existing site visitors to stay top of mind and drive conversions. As a next step, it would make sense to target sites and data sets where the target audience is found. Think about the demographics and interests of the target audience for your advertising message, and scout sites accordingly.