Why not? Or why not use both?
> The primary use case for these is for blocking ads on devices that don't allow running a real browser and yet still shows ads, such as "smart home" devices, TVs, etc.
What about non-browser apps on mobile devices or even desktops? Lots of apps have invasive ads and are unlikely to offer an extension api to block them with.
Because DNS-based blockers aren't visible to the browser, so they just look like HTTP errors or worse, and cause a variety of misbehavior. They're much more likely to produce errors that feel like the site just doesn't work. They can't distinguish between requests to different URLs on the same server, and many sites distribute both ads and content from the same servers. So they're always either going to miss ads or break sites, or both.
Browser-based blockers can block some URLs while allowing others, in addition to many many other improvements like substituting no-op scripts for things the site expects to call (preventing sites from hanging because they're waiting on tracking, for instance).
> What about non-browser apps on mobile devices or even desktops?
Ignore "download our app!" prompts and stick with mobile websites wherever possible; Firefox Mobile has excellent adblocking via uBlock Origin. Look for ad-free alternative apps. If that isn't an option, purchase ad-free paid apps.
Simple answer: don't use those apps. Do you really need them?