Dialogue is one of the writer's best means of 'showing, not telling' the story.  It engages the reader's interest as exposition and narrative rarely do.  Alice, looking over her sister's shoulder, who is reading a book with neither conversations nor illustrations, grows bored enough to fall asleep and wander into Wonderland.  It is not only important to write dialogue, it is essential to write it right! 

While it may not always be a good idea to write the way you really talk, it is essential to write dialogue the way people really talk. 

Real people--listen to them!--speak with a total disregard for the rules of the written word. They speak in broken sentences; begin with But or And; their sentences run on and trail off, and they never ever use punctuation, because they have tonality to do that job. 

Real people use conjunctions. Or they don't. What kind of people use 'em? What sort of people always say every word, every syllable, very, very clearly? 

Real people have little verbal mannerisms, eh? I know, right? 

Real people make up words that are not even words--yeesh! 

Real people misuse words and disregard grammar: Yeah, me and Yogi, we shoulda stood in bed. They say mischievieous just as if it's a real word. 

Sometimes theytalkreallyfast. Some times... they... talk... s l o w... And sometimes they Emphasize. Every. Damn. Word. 

In conversation, people interrupt each other. 

When you write dialogue, forget the rules of writing narrative: write speech the way people speak. Write your characters's speech the way that character really would speak. 

Through dialogue you not only convey information about what your characters have to say, you reveal your characters: who they are, where they come from, what they are like as people. You also fill in the scene without weighing it down with narrative: 

"Turn up the music so we can hear over the road noise--I have to keep my hands on the wheel. Would you look at all the traffic? You kids back there, don't you drop those cones! Geeze, the weather hits 90, everyone has to go to the beach at once! Not that station, you know I hate that kid stuff... put it on my station." 

"But, Mom...!" 

“Don't take that tone, put it on... Geeze! IDIOT...!!! Did you see what that idiot did...? He coulda killed us! Geeze! Guys in their hot little red cars...! No, put it on country, and remind me to remind your dad to get this damned a/c fixed, it doesn't reach the back seat, let alone the way-back." 

So what do we know? Mom is driving the kids--at least three, one up front likely a teenager and the ones in the back eating ice cream--to the beach on a really hot day, in terrible traffic. It's an older station wagon or minivan with faulty air conditioning and Mom is frazzled. Oh, and she likes country music.

I'll bet your own mind and life-experience fills in still more than that.

Use dialogue to 'show' your story and characters; give your reader enough to trigger their own inner vision, and use your words in your characters' mouths to guide that vision where you need it to go.