As you have already noticed,
Builds (mostly) are for PvP or GvG. You have already stated that Builds for these are to target certain characters or builds (called Anti-...).
The samethine applies in PvE. There are builds that are made to quickly and easily take down certain monsters. Monsters have a set amount of defense, damage, and skills. Therefore, they are easy to predict what they have and what they can do. So, people create builds to easily dispense with those monsters. The reason for this is so people can farm and gain XP or money or items.
Can you take down monsters without a build? Absolutely, but some people have a hard time taking down 5 griffons, or 20 trolls, etc. Some people have taken time and effort to create an easy way to destroy certain monsters and have graciously provided their knowledge to others.
I myself, don't use builds, so sometimes I get my butt kicked by monsters, and sometimes I crush them. Some builds, like runners, are created to easily move people around areas or missions. They "taxi" people to their destination for either "tips" or a straight fee.
Builds are basically used to do certain tasks easier than others.
So, you don't need a build to do things, just sometimes, its easier with one than without (in certain situations).
Hope that helps.
