A while ago, I was watching a movie with My Twin Formerly Known As The Other One, and when the villain was on the urge of admitting that they were the Vile Betrayer, my sister said, "It'd be great if that character had reasons and motivations, instead of just being a bitch."
After about half a moment's consideration, I realized she had a point. We were watching a character betray everything they were supposed to value and stand for, and they never actually explained why. No one even so much as implied that he'd been offered vast sums of wealth in exchange for his efforts in this villainous plot.
Now, I don't know about most people, but I feel I'd certainly need a reason to suddenly start working for the powers of evil and darkness. In fact, generally speaking, I like to have a reason for pretty much anything I do. And while I won't cast myself as an expert on the human race, I'm going to go out on a limb there and say most people are the same way.
People have motivations. Characters, as mirrors of people, should have motivations as well. Tempting as it might be to have characters charge blindly in a certain direction simply for the entertainment value (and, I know, sometimes it's tempting), characters need to do things for a reason, otherwise your reader's going to be sitting there wondering, "I don't understand. If she has awesome control of the universe, why didn't she just cause the chocolate to appear before her, instead of going on a magical quest to Hershey, PA?" And then your reader's going to have a point. (I know, dashed inconvenient, isn't that?)
Plus, motivations keep your characters interesting. A well-motivated character will be able to do interesting things with impunity, because they'll make sense. And a well-motivated character is usually more interesting, because motivations derive from important events in a character's life, and these events flesh out the character.
Motivations are the whys of characters and their actions, and understanding these whys helps the reader understand a character's actions and you understand your character.
In my WIP, Cordamant's Heir, my MC, Amira, has to flee the country to escape murder charges. She flees west (the direction of my plot), instead of north. Why? Because she's got slight issues with, well, the entire country to the north. Why? That country instigated a war that led directly to her father's death. So she's not exactly interested in moving there for an indefinite period of time. Does she know this prejudice is unfair? Yes. Does she know this isn't the time to let her thinking be influenced by irrationality? Yes. Does she let herself be influenced anyway? Yes. Why? Because this is too deeply ingrained an emotion to be combated by simple logic alone. She's heading west, and she's doing it for her own reasons.
In Harry Potter (because life is also improved with a Harry Potter reference, and because I feel reasonably sure I can make it and be understood -- ooh, look, I have motivations), character motivations are very important. There's a running issue throughout the series based on Professor Snape's motivations for saving Harry in book 1, because Harry thinks Snape's motives are one thing, is told they were another, realizes he was in error about the other, and in the final book realizes he was just dead wrong about all the reasoning. But without Snape's being motivated to keep Harry alive, Harry would have died in book 1, and there'd be no other books, and wouldn't that be sad for all of us? Oh, and Snape's motivations for keeping Harry alive were also his reasons for being in the Order in the first place, and for teaching at the school, and for hating James, and for serving Dumbledore so faithfully. Gosh those motives were integral.
Motives are useful. They explain your character and their actions to both the reader and the writer. They create the internal logic of the story, so that things don't seem to be happening willy-nilly. They are the answer to the ever present "why?".
Do you ever find yourself reading/watching/listening/observing a situation in which a character has no distinct or readily apparent motivation but instead seems to be acting at random? How does this make you feel? How far are you willing to follow a character of no apparent motivation?