Harnesses are typically not designed for helping your toddler learn how to walk. They should be walking on their own and that is usually when they are most useful. Young children who have just gained the skills of walking and then running on their own often do not want to be restrained from this newfound freedom. They also may be too young to understand how dangerous the world is around them or know to avoid hazards such as busy streets and edges of cliffs. They are spontaneous and can be easily distracted putting them at risk of running or being left behind. Constant vigilance is required at this age and can be exhausting especially when you have competing needs or are traveling to new, unfamiliar destinations. As your child increases their skills and ability to stay close by and listen, you can decrease how often, what occasions, and how long the harness is worn.
Children with special needs may take longer to learn the skills needed to walk safely in different environments and harnesses may be helpful for longer periods of time.