Why is it important to use positive guidance with young children?

Because hitting them with hammers is illegal

So you'll establish a warm relationship between you 2, a true parent-child bond instead of a colder "I know what's best and you'll do the way I say" at least this is what I think is the reason

Because you want them to have positive ways of looking at themselves and situations. If you only use negative reinforcement, you will have negative grownups in the future.

To establish and foster trust which will breed respect for boundaries.