It's really nice for random browsing/apps. Games, free tools, general web browsing; none of it loads ads.
Some mobile games will even attempt to load ads, fail, then give you the reward for 'watching' the ad.
It also stops devices from phoning home to upload telemetry and blocks known malware domains. (depends on the lists you use, heres a source for some lists)
I can tell you something: I'm using some free apps on my Android phone and never notice ads at home, but when I'm on the mobile network or on a different wifi (at work or some public place) I start seeing them.
This is one of the reasons why I set up a Wireguard VPN connection to my home network, and an on-demand VPN connection on my phone that automatically turns on anytime I’m not on the home network. Even away from home I get the benefits of my Pi-hole+Unbound (running as recursive resolver) setup.
This is my setup too. I use WG-Tunnel to manage the VPN connection on my phone. It just monitors whenever you disconnect from your trusted WiFi network and automatically enables the VPN.
Only hiccup I've found is wireless Android Auto isn't a fan of a VPN.
I have both but just use pihole as a local DNS server/forwarder. I bump into too many random times where sites or redirects don't work properly since they get blocked.