In many schools and families, just letting kids make a mess is considered risky. That’s why a good kit can be the stepping stone to trying something outside your comfort zone. All the parts and all the directions are there for you, plus (hopefully) background information that will teach you something about what you’ve made. While it may involve a mess, at least the odds of ending up with something cool are good.
Open-ended kits are ideal, but even kits that steer you towards a particular end product are useful if they encourage the child to extend their play once the first goal is reached, or inspire you to keep going using your own materials. Here are some kits I saw at New York Toy Fair that fit the bill.