Why use them? Similes put clear pictures into people's minds, and they make your writing more interesting. Look at these pairs of sentences. Which sentence of each pair puts a clearer picture into your mind?
The man is strong.
The man is as strong as a bull. (The simile in this sentence compares a man to a bull. You're not saying that the man weighs 1500 pounds (700 kilograms) and eats grass. You're saying that the man's strength is like the bull's strength.)
It was dark, and the lake had no waves.
The lake was like a sheet of black glass under the moonlight. (The simile in this sentences compares water to glass. You're not saying that the water was hard and fragile. You're saying that the calm water looked just like a sheet of glass.)
Examples: like similes
The night sky twinkled like diamonds on black velvet.
His hands were so cold, they were like ice.
The planes flew overhead like a flock of birds.
Examples: as similes
The ballet dancer moved as gracefully as a swan.
The frozen steak was as hard as a rock when we took it out of the freezer.
My son can be as stubborn as a mule sometimes.
I'm as ready as I'll ever be. (There's a few "as" words, but nothing is being compared, so there's no simile.)
My girlfriend looks just like her mother. (There is the word "like", and you are making a comparison, but you're comparing a person with a person, which are two very similar things, so there's no simile. In order for there to be a simile, you have to compare two different things, like a person to a tree, or a happy face to bright sunshine, and so on.