Ad ARPDAU isolates the advertising side of daily yield. It decomposes neatly into two levers: how many impressions each active user sees, and what each thousand of those impressions earns.
That decomposition is what makes it useful. A rise driven by impressions per DAU usually means a placement or frequency change, which can carry retention risk. A rise driven by eCPM usually means auction, demand or geo-mix changes. The two call for different follow-up checks.
Read it by format (rewarded, interstitial, banner) and by country tier. A blended ad ARPDAU hides the fact that a Tier-1 user and a Tier-3 user can differ by an order of magnitude.