To get a unified picture of how individual users engage with your content across platforms and devices, you first need to implement User-ID.
User-ID depends on your ability to generate unique, persistent IDs and assign them to your users so those users are counted only once when they engage, for example, first via your Android or iOS app on their phones, then later on your website via a tablet or desktop.
Deduplicated user counts are helpful when you're looking at overall numbers of users, and metrics like New users. Without User-ID, users are counted as new users each time they open your app or site on a new device, or every time after they clear their ad identifiers or cookies. With User-ID, though, those users are counted only once as new users.
Users are not deduplicated under circumstances in which they can be counted legitimately in multiple categories. For example, if you have separate audiences for female users and users who are 18-24, then any female who falls into that age category will be counted once in each audience.
Deduplicating users does help with audience creation, though, to ensure users are counted only once when appropriate. For example, if the same female user opens your app on an iPhone and on an iPad, you want to count her only once in the female audience: if you're using that audience for remarketing, you don't want to overexpose the same users to ads, and you don't want to waste ad budget buying twice as many ads for the same user.