If you don't, bad things will happen to you. Bad things like this.
I tried to set up a lava pool in the basement, not only as a source of light, but also as a place to burn unwanted items. Unfortunately, I didn't set it deep enough into the floor. So, when I put the lava in it, some embers floated up and set the main, wooden floor ablaze. The fire then spread to the upper floor and finally to the roof.
Initially, I tried to control the fire by covering the lava with cobblestone, but that just caused the lava to spread and did nothing to stop the flames from consuming the floors and the roof.
So, I panicked, and tried to put out the flames by pouring water on them. As you can see, though, I lost control of the water, too, because Minecraft-water doesn't behave like real water. Eventually, I just gave up, and decided to let it burn. Oddly enough, that torrent of water stopped the spread of the lava, and gave me the opportunity to cover it up and put out the fire. However, not before it gobbled up much of the roof and the floor and claimed one of my cats. The poor thing dropped from the upper floor right in front of me, all ablaze and screeching with pain. I had to get out my sword to put it out of its misery. Sure it was only a blocky Minecraft-cat, but it made me feel like a horrible person.
After the disaster, I quit the game in frustration and decided to clean up the mess at a later time, when I had a clear mind. I have done this, and I made some changes to the interior of the house; I'll put up those pictures in another post. Until then, use lava responsibly, and learn how objects and elements behave before you try manipulating them to suit your needs. Otherwise you may end up destroying your house and burning your cats.