I can't tell you how many times my younger dancers have asked this question (in a variety of ways... mostly some variation of "Ballet barre is boring, why do we have to do it?!").
If you look at any sport or art form, you will see that they will do their own version of the ballet barre: a singer will sing scales during warm-up, soccer players run drills - my students understand this metaphor (even if they don't like the result: more ballet barre).
Barrework allows you to focus on minute sections of your body that when you perform, will be (hopefully) more automatic because of your preparation at the barre.
If it wasn't so important, then why do professional ballet company's ballet barres begin with the same preparation the youngest beginner-level dancer does: plies and tendus? These are the building blocks to your technical skills.
Henrik's reply on the blog said it best:
"Barre work is...the place where the necessary skills for freedom of expression are built."