Depends where in the world you are. In the US it was popularized by Dr Kellogg to curb masturbation. He also recommended a few drops of carbolic acid applied to young girls clits to damage the nerves and avoid what he called "abnormal excitement".
Dude was opposed to pretty much anything even vaguely resembling pleasure, he invented corn flakes as a food to be as bland and tasteless as possible. The only reason they ever became a popular breakfast cereal is because of his brother adding sugar to them despite Kelloggs objections
'cleanliness' and 'looking like their father' were later justifications after the practice had already gained traction.
The original post only gave half the explanation. It's not that lead exists in general, it's that lead exists within zircon crystals.
Under normal circumstances that would be impossible, zircon crystals strongly reject lead atoms as they form. There's no way to stuff lead into the crystal lattice in the quantity we find them there. But uranium and zircon go together just fine, we just have to wait for it to decay into lead. The trouble is it takes ~4.5 billion years for just half of those uranium atoms to turn into lead. So any zircon crystal we find with half as much lead as uranium must be roughly that old