Here's why grooms usually stand on the right and brides on the left during a wedding ceremony.
Long ago, the right arm was considered the sword arm of most fighting men. If a man had to protect his bride, he would hold her with his left hand, and fight off attackers with his right arm.
The reason that men may have had to fight off others was that quite often women were kidnapped. Family members naturally wanted to rescue the stolen brides. Sometimes even during the wedding ceremony, the grooms had to fight off other men who were desirous of their brides, along with the bride's family members. So having his right arm free was an important strategy.
This tradition is followed today by when facing the officiant, having the bride stand to the left, and the groom stand to the right.
Another reason is that the union of the right hands of the bride and groom symbolizes their union as one, their strength in being together, and the combined resources that they bring to their marriage.
Exceptions to the Tradition:
During a Jewish ceremony, the bride is often on the right, and the groom is on the left. Other exceptions you may see are military and police weddings.
And even though you may see the bride standing on the left and the Groom standing on the right, more and more you will see signs that invite guests to "choose a seat, not a side." This is because more and more couples share a friend group and don't want to put those friends in the position of having to choose.