Why do we give out candy on Halloween?

The practice of children dressing in costumes and going door to door for candy didn't start until the 1930s and early 40s. In England there was a practice called "souling" where poor people would visit the houses of the rich, promising to pray for their deceased family members in exchange for "soul cakes." In Ireland and Scotland, there was a custom called "guising" where people would dress in costume and go house to house telling jokes in exchange for a treat.

Immigrants brought these traditions to the US in the late 30s, and as trick-or-treating became more widespread as the years went on, candy became the primary treat as it was easy to buy and distribute, and posed a smaller risk of poisoning/tampering than unwrapped treats. Candy manufacturers began making smaller versions of their product and by the 1970s, candy was really the only "acceptable" treat to hand out on Halloween.