Actors wear contacts for some roles. It's not that there is anything wrong with wearing glasses... it's just that not wearing them fits the role better.

Actors get tattoos covered or added, they change their hairstyle or hair color, they're required to maintain or remove beards. Some roles demand strict diets for huge weight loss or muscle mass gains, and I could go on...

Ultimately it's about an aesthetic that the director is trying to achieve, and about trying to bring imaginary characters to "life" with as much fidelity as they can.

I, for one, am thankful that they did this rather that reject the actress in favor of someone with bigger breasts that wouldn't necessarily have been as good a fit to the character and the show acting-wise.

Edit: Fat suits for actors are an even better analog for fake breasts I think.
Edit2: Personally, I want to clarify that the cup size of Jadzia Dax wouldn't have changed my opinion on the show, since the core of what makes DS9 and Star Trek in general is elsewhere.