I block all sorts of stuff thats not even ads. Stuff like "suggested for you" sidebars on websites I frequent I will just right click and block. All sorts of stuff. If I don't use it, no need to load it next time, so out it goes.
DNS-based ad blockers only block connections to domains known to serve ads.
A browser-based blocker is able to block ads served by the same domain as the content you actually want, without blocking said content. They are also able to apply fixes to pages that break or have bad layouts when ads fail to load in.
I once sent someone a YouTube link and they were wondering why I sent them a link to an ad. It didn't even occur to me there would be an ad there because I was using FF and UBO, and obviously he wasn't. UBO just makes it so seamless that it is easy to forget.
UBO catches things for me that Adguard doesn't, so having both is pretty useful to me.
UBO also blocks harmful elements inside of a page, not just page loads. I've used it to automatically remove the annoying paywalls that are just an element over the text I want to read.
Maybe you are confusing AdGuard with AdBlockPlus? The latter[1] and the parent company Eyeo GmbH have some points of criticism.
AdGuard is basically pi-hole, just with different pre-configured filter lists and especially child and parental control settings. IIRC, some services can also be blocked there directly, Instagram for example.
the simple improvement in internet speed is well worth it, even though i do too have a pihole running.
Sometimes when i visits friends i barely recognize sites that i use frequently.