It would give you access to the host file
Before anything connects to the internet, it checks the host file to see if the address is already located there. (Why download it again if it's already stored locally?) So one can use the host file to list all the ad networks you done like and POOF all those ads are gone (unless you're actually hosting them from your phone, but you're not so it will return null)
This, you don't have to even run a program. Every phone uses the host file all day all the time anyway, you're just telling it to load nothing instead of usual behavior of "no it's not hosted here, go get it online" you're just saying "no, don't go get it, it's hosted here" except it's not, so it isn't retrieved at all.