Three Big Roles
1. Help Plan and Pay For The Bridal Shower and Bachelorette Party: There are going to be many different celebrations/"experiences" leading unto to the day of the wedding! Two of which are the bridal shower and bachelorette parties. It is expected that the bride's main girlfriends aka bridesmaids are responsible for planning and footing the bill. During the planning process it is common that the bridesmaids will hire a professional to assist with the concepts and orchestrating of these events.
2. Help Select & Purchase Bridesmaids Attire: Yes, ladies you're expected to purchase your bridesmaids dresses! However, your in luck these days, because the days of the hideous puffy sleeve dresses are long behind us. It's so common today to have an equally stylish BM dress as it is a bridal dress.
As well as paying for your attire you can also expect to pay for your makeup, hair and nails to be professionally styled. I mean it is your best girlfriend's wedding day so, you have to be prepared to SLAY!
3. Attend The Rehearsal & Rehearsal Dinner: Not only should you attend the wedding rehearsal and dinner, but you should be on time! In most cases the rehearsal is held a day or two prior to the wedding and only lasts for a hour. Most venues only allot a hour and most coordinators are contracted for a hour. So, it's in very poor taste to arrive to the rehearsal late.
Also, once you arrive, be polite and listen to the coordinator's directions. This isn't the time to offer suggestions on "how you did it or how it should be." Offering suggestions often prolongs the rehearsal; I'd only offer a suggestion when asked to specifically by the bride, groom, or coordinator. Most times the couple have already discussed with the officiant and coordinator as to how they want their processional and ceremony to proceed.