It defines a generic mechanism for performing actions that don't fit the GET/PUT/PATCH/POST/DELETE model on resources and collections. For example aggregations:

  POST /orders?action=sum&field=price HTTP/1.1

  HTTP/1.1 200 OK
  Content-Type: application/json
  {"sum": 9000}

Or actions with side effects:

  POST /orders?action=process-queue HTTP/1.1

  HTTP/1.1 204 No Content

Which is better than creating a specific resource for each of the "actions", such as /orders_sum_price or /orders_process_queue, because it keeps the URIs clean.