The point of making your kids wear a diaper is to keep the kids clothes clean and also to keep your furniture, carpets etc clean.
If a kid who is not potty trained does not wear a diaper then they will pee and poop in their pants and get it all over them, the bed, your couch, the floor, in your car etc.
Without a diaper on the kid who's not potty trained there would be a huge mess and it would be unhygienic and unhealthy for the kid to be peeing and pooping on themselves and in their clothes.
When babies are born they have no bladder or bowel control and even as the child gets older and reaches 2 to 3 years old the child's bladder is still weak so they can't hold it very long.
So a child who is wetting their pants or pooping their pants or those who are not potty trained will need to wear a diaper.
Even if the kid does not want to wear the diaper they must be made to wear the diaper if they are not potty trained.
Or if a kid is incontinent or bed wetting then they should wear a diaper to keep them healthy and clean.
Some kids potty train by age 18 months and some are not ready until 2 to 3 years old.
Then again some kids may not be potty trained until 4 and even 5 years old.
Every kid is different and will eventually potty train and want to stop wearing diapers but until then the child must and should wear a diaper.