Some cat owners think that their female unsterilised cat should have a litter of kittens before they sterilise them because it helps them to settle down and live a more contented life.
They appear to believe that a female cat should have kittens because it's instinctive to them to have kittens and if you deny them that pleasure it is upsetting to them.
Until they have kittens they can't settle down and be their normal selves. That is the argument as I understand it but it is entirely false and dangerous thought for the obvious reason that there are already too many unwanted cats in the world and to bring more into the world because of a misconception is irresponsible and ultimately cruel because what happens to the kittens? Can you find a good home for them? Will they be happy? Will they have good lives? Will they be homed with a person who is sensitive to their needs and so on?
It is a myth that having a litter will settle down a female cat. It's a myth that the cat will feel more fulfilled after having kittens.
It might be useful to touch on how female humans feel after having kids. That might provide us with a little insight. Mothers are often harassed and overstretched and pressured on having a young family. They certainly don't feel more settled down.
The veterinarians state that a spayed cat will be more playful, people-orientated and they won't miss what they never had.
Briefly, on an associated topic, there are other reasons why cat owners sometimes want their female cats to have a litter of kittens. It might be because it provides them with enjoyment. They'd like to witness the birth of kittens. They might enjoy raising them and find the whole process rewarding.
They might love the appearance of their female cat and they want to produce offspring to maintain the bloodline. Or they might do it for financial reasons to sell the kittens for whatever they can get for them to make a few pennies on the side.
If the cat is a purebred then they can sell them for a reasonable sum of money but the amount would be dependent upon whether there was a registered lineage with a cat association.
That said you will find some cat owners advertising on Facebook purebred cats for sale at discount prices. Don't buy these cats and kittens at the prices asked. You are probably not going to buy a genuine purebred cat and I would expect it to be unregistered.
And lastly, some cat owners may have a deep emotional bond with their female cat and want to have the experience of raising a kittens to create a stronger connection with the cat and their offspring.
All these reasons may be good ones from the person's perspective but in the grand scheme of things, I think it would be fair to say that it would not be the best decision to make in terms of general animal welfare for the reason stated above.