"Why does it matter?"

It matters! In fact it matters to matter so much that you probably wouldn't exist without the existence of black holes.

A Black Hole is a place in space where there is so much gravity caused by matter that is so compact that even light cannot escape.

"How did they get there?" 

Thanks to Einstein's General Relativity we can understand that a sun will distort space time around it more and more the more compact and massive it is.

Once that star begins to die and run out of its nuclear fuel, it gets cold and shrinks, leaving a bottomless hole in space time.

(If you don't understand GR, worry not! There will be a post about it.)

The early universe right after the big bang was probably full of many black holes.

They might help give birth to galaxies! That's cool!

"How does a destructive black hole create stuff?"

The secret is in Quasars;

They are the brightest light in the universe and can outshine entire galaxies!

A black hole at the center of a young galaxy sucks up a lot of gas. Once it fills up too much and all the gas becomes heated due to its fast movement, it lets out a burp (because gas makes you burp) called a "Quasar" and spits out the gas. The quasar will run out of gas to feed on and eventually cool down and turn into a supermassive black hole. Black holes and Quasars are key to understanding how galaxies form! Scientists have recently discovered that black holes that turn into quasars and release huge jets of energy have actually helped create stars. So now we must ask if quasars help create all galaxies or do galaxies create quasars? Right now the answer seems to be yes to both... until further notice. 

In my humble opinion:

(My thought is that all galaxies have black holes at the center, which create quasars, which heat other parts of the universe and help create stars, which may turn into black holes and create new galaxies. Sort of like a space reproductive system!)

"Can they kill us?"

Sure! If you get really close to one and pass through the Event Horizon, the part of your body that's closest would have a huge gravitational pull much larger than the other half of your body... turning you into the literal flying spaghetti monster. Then the pull of gravity would increase as you approach so much that your body would just be ripped into tiny particles. If someone were watching you from afar, they might not be able to see you actually go into a black hole. You would appear to be slowing down and hovering the invisible mass.

But that's not going to happen. Don't worry. 

In fact there is a supermassive black hole at the center of our own Milky Way Galaxy! 

"How do you know?"

Scientists use powerful telescopes that by using knowledge from discoveries in optics, can detect light and other energy invisible to the naked eye. They can look to the center of our galaxy and see that things are moving really fast around a single point, since black holes are invisible, we have to observe the matter and energy around one in order to notice they are there, unless there is a quasar we can see.

Common Misconception: "Black holes are giant cosmic vacuums!"

False: The truth is actually much weirder than that. You'd have to be within 10km of a black hole before you'd even start spiraling into it.
In fact if our sun was replaced by a black hole of the same mass, the orbits of all the planets would not change.