It's surprising, but yes, it does actually make a difference and add that much flavor. If you replaced it with fennel, you would get an entirely different flavor. If I lived in the U.K., I would show you a side-by-side of a sauce made with a normal mixture, and then one with fennel, you would immediately taste the difference.

Although personally, I definitely agree with you that by itself celery is pretty meh. But in terms of cooking with other things as a whole, it's important.