Engelsiz DNS Changer App: How does it work?

I found an app called "Engelsiz DNS Changer" on Google Play. It changes the DNS settings on Wi-Fi and 4G without root. I just started using it after labor day. I noticed that my data usage to date (after starting use of the app) is less than 25% of my monthly usage. On average I use over 50% and sometimes closer to 75% of my data per month. I'm sure this Engelsiz has something to do with it. I wrote to the developer to get an explanation on how his app works, but he couldn't really explain it in terms I could understand (He's from Turkey).

I'd appreciate some help from the XDA community to see if this app is something I should be concerned about of if everything is legit.

Thank you :)