The Latin for lead (the metal, not the advantage) was plumbum, and so there is an obscure English word plumbous, which means leaden or heavy. But the uses of lead are various and so, therefore, are the words that derive from it.
The main thing about lead is that it is heavy. If you attach it to a piece of string, the piece of string will hang down perfectly vertically. It takes a strong wind to move lead. What you therefore get is a plumb line, which, combined with a right angle, was terribly useful in building until the spirit level came and took its job away.
It's also useful in working out the depth of water, as currents will not sway it much. Therefore one can plumb the depths using a plumb line. The weight will plunge into the water (from Latin plumbicare) and then plummet to the very bottom. The line will remain perfectly upright, or as the French would put it à plomb. If a person were to remain so upright in the currents and whirlpools of life, they would therefore display perfect aplomb.
But the other use of lead is that it's wonderfully soft for a metal. You can work it with your hands and it was this advantage that meant that it was used in roofing and then in Victorian water pipes. So universal was the use of lead, that by the 1870s piping was simply known as plumbing. Those who worked in lead had been known as plumbers for centuries, but now they became the people who fixed your taps.
The other thing about lead is that it's poisonous.
The Inky Fool wanted to be alone in his bathroom.