Through history.. the bridesmaids & groomsmen have had many different responsibilities.
This comes from MentalFloss.com:
1. In a time in which “marriage by capture” was practiced, close friends of the groom would assist him in taking the bride from her family. They’d form a small army to fight off angry relatives so that he could escape with her.
2. Witnesses at the marriage bed were once required to get REALLY involved. A tradition in medieval England and France was called "fingering the stocking”: literally checking the bride's stockings for signs that the marriage had been consummated.
3. There was a lot of shoe-throwing in the old days. In Anglo-Saxon times the groom “symbolically" struck the bride with a shoe to “establish his authority.” Brides would throw shoes at their bridesmaids (instead of a bouquet) to see who would marry next. Whoever caught it would throw her shoe at the men, and the first guy hit would be the one to wed.
4. Ancient Roman law required 10 witnesses to be present at a wedding, which is considered a precursor to the bridal party tradition. Bridesmaids and groomsmen had to dress just like the bride and groom to confuse vengeful spirit presences (or real-life jealous suitors) who might try to harm the newlyweds.
5. Another origin story for the bridesmaid tradition is Biblical: When Jacob married Leah and Rachel, each brought her own “maid”—but they were personal servants rather than your typical bouquet-holding bridesmaids.
6. The tradition of the “best man” is thought to have originated with the Germanic Goths of the 16th century. He was the “best man” for, specifically, the job of stealing the bride from her neighboring community or disapproving family, and he was probably the best swordsman, too.
7. In some early traditions, the groomsmen were called Bride’s Knights, because they helped protect her—and her dowry, and her virginity—or because they assisted in her kidnapping.
8. The chief bridesmaid might be in charge of the dow-purse (much the way today’s maid of honor would hold the bride’s bouquet). She’d also help the bride take off her gloves and then hold them during the ceremony.
9. In some traditions, bridesmaids led the bridegroom to the church and the groomsmen led the bride.
10. In medieval times, some bridesmaids made the bride drink and eat a concoction of plum buns in spiced ale to “restore the energies.”
11. Part of the job was to walk carefully: If a bridesmaid stumbled on the way to the altar, the superstition was that she would never marry.
12. Given the likelihood that the bride's family would attempt to retrieve her from her groom or get revenge—or that another suitor would try to take her, or she might try to escape—the best man stood right next to her at the wedding, at the ready with his weapon. Later, he was moved to the groom's right side (possibly due to jealousy on the part of the groom). After the ceremony he stood guard outside the newlyweds' bedroom or home.
13. In ancient Roman weddings, the matron of honor was a moral role model, known for fidelity and obedience. (She had to have been married no more than once, and to have a living husband.) She joined the right hands of the bride and bridegroom for the first time at the ceremony.
14. In early Victorian times, tradition called for all-white weddings, so bridesmaids—who were supposed to be younger than the bride—wore white dresses with short veils, contrasting with the bride’s more ornate veil and train. By the 20th century, this had fallen out of favor, and the bride alone wore white to better stand out.
15. Victorian bridesmaids were tasked with making party favors out of things like ribbons and flowers and pinning them onto the sleeves and shoulders of guests as they left the ceremony. Bridesmaids of the past also used to walk down the aisle with aromatic bunches of garlic, herbs, and grains to drive evil spirits away (and to help make things smell nice in times when hygiene was a bit different).
16. A maid of honor once attended to the bride-to-be for several days prior to a wedding, making sure the bridal wreath was made and helping her get dressed. Bridesmaids also helped undress her, making sure to remove all pins (if a pin remained, it was bad luck for the wedding), and helped decorate for the wedding feast.
17. The “stag” or bachelor party originated in Sparta in the fifth century, as his buddies—de facto groomsmen—toasted him and feasted on the night before his wedding.
18. Being a bridesmaid was considered a good way to procure a husband. In the 16th century, if you had served as bridesmaid three times without getting married yourself, it was believed that evil spirits had cursed you. To break the spell, you’d have to be a bridesmaid four more times, for a total of seven rounds on the wedding circuit.
19. The bride’s friends would “shower” her with gifts before her wedding in cases when her father didn’t approve of her groom and wouldn’t provide the necessary dowry for her to marry the man of her choice. The gifts they gave would become her dowry.
20. The bride was often accompanied by a child—think today’s flower girls and ring bearers—meant to symbolize a fruitful union. Flower petals tossed in the bride’s pathway were representative of the way to a beautiful future.
21. Open carriages were considered an easy target for evil spirits, so wedding guests would use bells and firecrackers to scare them away. This translates to today’s celebratory car honking after ceremonies.