The Wheelbar's primary function is to help Penny get around Dr. Calico's bases and unlock their secrets. She can use it as a zipline or a hook on lines and can hook it up to pipes and other devices to get herself up to places. Penny can also use the Wheelbar to open doors and activate devices like waterwheels and even the mechanisms for the ancient temple.
Outside of these, Penny can extend the Wheelbar as a weapon to defend herself against Calico's henchmen when Bolt isn't at her side.
The Wheelbar is a bit like Penny's scooter in the film.
Penny wielding the Wheelbar against one of Calico's henchmen