The word adopt is used in a number of contexts. For most people, it means to become the legal parent of another's person's biological child.
International aid organizations such as Save the Children have long known that humans are more moved by the needs of one individual with whom they have an emotional connection than by the overwhelming need of a group. The tradition of sponsoring a child is designed to parallel the adoption experience, with sponsors receiving photos, letters, and updates on the child they have chosen to "adopt." Sponsors know that the money they send goes to support the community, not directly to "their" child, yet the concept works so well that the sense of connection and relationship are encouraged.
Pets are adopted, not purchased, from the pet store, emphasizing that this is an emotional relationship. Picking up on the theme, Cabbage Patch dolls - the rage of the early Eighties - came with a name and an adoption certificate. And who didn't yearn to adopt a Pet Rock in the Seventies?
In most cases, however, to adopt means to take a child to be your own - to love, care for, and support a child who is yours by law as you would a child who is yours by birth. Zoos and agencies and doll companies use the concept of adoption for the same reason it has lasted from earliest history: because it carries powerful emotional connotations.