training collars help you teach your dog to walk off-leash
fix a reliable recall in all circumstances
stop the dog from running away after game (commands "stay", "to me", "to my leg" and "stay"), other dogs or people
unlearning to pull on the lead,
interrupting unwanted dog activities ("yuck", "you must not") - unlearning bad habits in everyday interaction with people, e.g. taking food from the table, jumping on people, digging, eating excrement, unwanted barking, howling, etc.
precise training of practically every command, where the dog has to perform it as accurately and reliably as if it were on a (up to hundreds of meters long!) leash
the unwanted dependence on the lead is removed, which the dog recognises very well and adapts its behaviour to it