Basically, a wedding favor serves two purposes: It’s a memento of the event and it also serves as a “thank you, we appreciate your attendance”.
Did you know that wedding favors have been a part of wedding ceremonies for centuries?

In the past, wedding favors were not much different than today’s items, but not every guest was privileged (or allowed to) take home one. In the 16th century, wedding favors were a sign of extravagance, used to show off the family’s wealth; (only the rich could afford gifts or mementos for their guests. Some of these favors symbolized luck; sharing them with guests ensured good luck to all!).For example, almonds were once common favors, for they meant fertility, health, happiness and more prosperity

Bonbonnieres were also a popular favor (these were small, decorative boxes made of gold, crystal or porcelain, embellished on the outside with colorful and expensive jewels (!). Inside the boxes were “bonbons”(yes, the bonbons as we know them today).
I’m not sure about bonbonnieres per se, but many couples are currently returning to the tradition of giving away bonbons or other edible favors; they’re economical and guests love them!

Edible gifts can actually consist of anything that’s desired. The five most popular examples or choices are:

Chocolate-Besides gourmet chocolate, there’s an endless variety of chocolate-covered just about anything (!), among others.

Cake or Brownie Pops-These are small balls of cake or brownie mix made from cake crumbs and frosting. The balls are then covered in candy melts or chocolate and served on a stick. They can then be wrapped with clear cellophane or decorative ribbon.

Seasonal Treats-If a wedding’s right at the start of a certain season or on or near a holiday, the favors can be coordinated to fit that particular theme (chocolate eggs or pastel cake or candies for spring, caramel apples for fall or lemon (baked) treats for summer).

Cookies-There’s a world of ways that cookies can be made, designed and decorated, to fit practically any theme. And they can be bought in advance, for most will last at least 2 to 3 weeks.

Petit fours-These are bite-sized pastries also fit in well as wedding favors; they can be easily paired and put in small boxes as take-home treats. Bakeries and many online vendors can create customized specialties, if desired.

Source: “Edible wedding favors make a fine choice”-2013 Bridal Planner, The Vindicator