Metaphoricals are used to describe things (objects, situations, events) that are difficult to describe with just words. This can be the only purpose of the metaphor – a good description.

This is especially true for feelings. That's the most obvious cue for needing a metaphorical.

I feel like I'm trapped in a bad movie. ("Affair")

A metaphorical can also help the reader feel things better than a description just telling the reader. In the following, a teenage boy is trying to talk to a girl he likes, but he doesn't know what to say or do. The author could have just said that, but he wanted to help the reader feel what the boy was feeling.

Seconds ticked by. Soon it would be too late, or late enough for whatever I did to look really weird, like I was stuck in a time delay like when they interview someone half a world away and you have this ungodly pause between the question and the answer (Teen Inc., page 22)

Calling up the memories was like watching an old home movie and not recognizing myself on the screen. (Here We Lie, DeBoard, page 130)

Having divorced parents is like living in one of those claw machines at the pizza parlor; you're just hanging around, minding your own business, and then every other weekend you get plucked up and flung somewhere else. (Every Other Weekend, Summerfield, page 27)

(Notice how the author steps out of the metaphor for "every other weekend.")

Mood

1. It was working horribly.
2. It was working as well as a car with a missing wheel.

Those say pretty much the same thing, so the simile (#2) isn't a more accurate description. But it's more concrete and more vivid.

Metaphoricals can also be more exciting. In particular, they synergize with hyperbole for excitement (and emphasis).

"I'd just prefer something [for their room decoration] that doesn't make my eyes feel like someone has poured bleach on them." (Free to Fall, Miller, page 27)

Right now, the idea of sleeping till noon in my own bed for two whole weeks felt like winning the lottery. (The Black Butterfly, page 9)

The door was heavy with disuse as I stepped inside. (page 35)

but the [painful] image never scabbed over (page 37)

Sex and tears, like sun and rain, were never meant to share a moment (page 37)

Some books need mood more than others. Teen Inc. (cited above) is less concerned with mood, so it has sporadic metaphoricals. That book also faces a challenging task of description, so the metaphoricals are carefully done and help the reader feel what the main character is experiencing.

In contrast, The King of Lies (cited above) is very concerned with mood, and it's full of excellent metaphoricals for that.

Every webpage I have seen on metaphoricals makes them seem necessary. They can be valuable tools – for description or mood – but they're not necessary.

For example, The Old Man and the Sea is somewhat light on metaphoricals. To me, that fits the mood Hemingway wanted. (I wish he had tried harder to leave them all out.) Ender's Game is another excellent book that didn't need or have many metaphoricals.

Metaphoricals can also pull the reader out of the story. Chandler's vivid, goofy metaphoricals tend to do that. Another example of too many metaphors (three in one sentence):

And if my life wasn't enough in the toilet, I was now on the plane home, seated four rows ahead of a guy who looked like Sasquatch and was snoring like a bear in a cave. (Evanovich, Explosive Eighteen, pages 1-2)

As reader, I am trying to imagine what is really happening in this story, and instead I am imagining toilets, bears, and caves. It's a crazy image in my head. But imagining Sasquatch instead of a man didn't quite interfere with the story for me. ("In the toilet" and "on the plane" conflict.)