Given a long string it will make insertion into the middle of text fast. Have you ever opened a file in Atom, added one character, and the whole thing locks up? That's the use case this solves, among others.
I opened the file in intellij and it's performing admirably but there is noticeable lag when entering input. Keep in mind that features were disabled automatically due to the file size.
2MB may not be very representative for all files, but when having to jump into generated protobuf class files I've certainly wished my IDE could handle the load. I can't tell you what data structures these IDEs are using internally though.
And, again, the fact that features had to be disabled in intellij really says a lot. With more power, you can have more features, you can get feedback faster in your IDE, you can have more linters, more plugins, etc. That's a huge benefit to having fast software. Performance is enabling.