Y did Queen Elizabeth II and Prince Philip have children after Prince Charles?

It's based on the idea created by the British of having an heir and a spare. Thus, Queen Victoria had lots of Children to ensure at least one of them would take over after she had died. Likewise HM Queen Elizabeth II.

Y not? In recent decades the notion of "an heir and a spare" has been raised.

So that if by some misfortune Charles will not be able to become King of England, then the next in line would be.

Princess Anne is anything but useless, she carries out more duties and engagements than any other Royal, she is patron of countless charities and organisations and she is commander in chief of several military regiments, while still being a full time mother and grandmother.and she still looks beautiful while doing it all.

Not a very clever troll are you.

Same reasons anyone has more than one child.

Just in case he popped his clogs at an early age. I think the Queen must be a catholic, and didn't use contraception.

The interval between Charles and Anne before Andrew and Edward arrived has long been a matter for speculation but stuff happens. It is felt necessary to have an heir and spare but at the time Anne probably wasn't considered a 'spare' because of her gender. So perhaps HM and her husband felt it wise to try for another male. But why Edward, who knows.

Promotion of their dynasty. It is a mystery why they are the only famous Royals when they all have faces like spanked bottoms, half bald, of average talent. No one knows who the other Euro Royals are for some reason.

What would have happened if Charles had died before having an heir?

I don't of course expect you to understand because understanding this simple concept would involve having an IQ greater than a can of Coke.