The situation is so very circumstantial though. I think it isn't a general thing for parents to kick their children out at 18 and nobody I know of other than through work has been. But I can see a number of reasons why it may happen.
Once child benefit is no more, it costs money to keep someone in the house. If they are unable to help with costs, it can just be unaffordable for some. And if a single parent, adds another 25% onto council tax bills once there's a second adult in the house.
You could live in a tiny cramped house with everyone doubling up in small bedrooms, and younger children needing more space as they grow.
And of course there are many reasons why the child's behaviours and attitude cause them to be thrown out e.g. refusing to do anything in the house or pay any bills, smoking cannabis, bringing very undesirable people in the house, being abusive to the other members of the household or even harming them, smashing up the furniture etc, stealing from the house... the list can go on.
On the other hand you may have reasonably well behaved children and plenty of space and money. When the children start work, if they stay at home they pay something (even if a token, or just by offering to pay) towards their keep. They do their fair share of household tasks like cooking, washing & cleaning, and not just their own.
Or your child may have some form of disability which would make life much harder if they had to move out.
Everyone's personal situation is different, as are everyone's children. It would be impossible to say either way is "right" or "wrong".