Why do we use field resolvers in graphql?

Question

So I have been following Ben Awad's 14 hr GraphQL and React series and I came across field resolvers and I have been a little confused about their use-case. I understand that field resolvers can be used to modify a certain data in the server. In this particular case, he uses it to slice the text of huge user posts, so in this context I am guessing that fetching large chunks of data is time consuming as the data grows, so field resolvers can be used to modify appropriate data to optimise the server. Am i right? Because this makes sense to me and this is how i understand it. Is there more to what they offer or is this it?