So, other than bringing up a 1990's action comedy, what's the point?
Your characters' need jobs.
Yes, it seems like that would be kind of a no-brainer, but it's surprising how many new and aspiring writers (and occasionally even professionals) miss this simple detail.
Of course, if you're writing a variant of Lord of the Flies or Children of the Corn, you're not going to be too concerned about the characters having day jobs. Most of them are just kids after all. Even then, though, they're going to separate into groups and layers, with certain people being given certain things to do. Social tiers isn't exactly where I'm going with this though.
My point is, it's important in your character development and for the story itself, to know about your character's day job.
Even if, during the entire length of your story, the character never actually goes to work, it helps to know what he does. It establishes a fair bit of your character's skill sets, after all, they do need to be kind of good at their job and every job has requirements. An accountant, for example, will be good with numbers, probably adept at using a keyboard, and know a bit about tax law; while a gym teacher will likely be in good physical shape, organized, and have good leadership skills.
A character's job will also have some effect on their appearance. An office worker will look much more professional than, say, a bartender or a landscape worker. Think along the terms of hair length, and how they usually dress.
Of course, neither of these really impact what a character might do or wear in their free time, but most people don't generally wander too far from their routine, even when they don't have to follow it.
Knowing what kind of work a character does tells us more about that character and gives us another aspect to bring up in the story. Things like why that character works there, why they chose that occupation, and maybe an interesting bit from working there. Or, you could go into what drove them to work there, and their true feelings about the place and career they ended up with if they don't like their job.
It's the little things about the characters that we discover on the journey through the story that makes them relatable and thusly, that makes us grow to like them and care about what happens to them. Things like having a job they hate, or love, or how they poked a hole in the bottom of their co-worker's styrofoam cup so their morning coffee slowly leaked out across the desk as a practical joke.
Huh? I know, I know. It goes like this. If your story is based on any kind of world like the one we live in, there is one constant. Money. And to get money requires work. How much money one gets depends on the kind of work they do. Money is how your characters afford their rides, their clothes, their houses, and their vacations.
Yeah. So what?
How often do you actually considering while writing, whether or not your character's job will actually provide for the lifestyle he has?
Your characters are in college, and can afford a month-long trip to Brazil on summer break, complete with a fancy hotel and boozing it up day in and day out. Are you really going to just go with the tired cliche` of the super-rich parents?
A struggling artist decides to go on holiday in Europe, traveling through Paris and Rome looking for inspiration. Not if they're actually struggling.
who wants to go to Paris anyway?
Get the picture?
It helps the plausibility of the story immensely when you can explain how your characters can afford the things they do. Even more so without the stand-by's of the super-rich parents or the secret agent with the credit card that has no limit. I'm not saying go into extreme detail, laying out their checkbook or budget or anything, but keep in mind, you might lose some readers if you have some kid that's six months out of high school flashing hundred dollar bills out the window of his super-souped up ride, and nary a mention anywhere in the book of where the money comes from.
We writers ask a lot of our readers for their suspension of disbelief, whether it be shadow demons in closets, true love on the Mekong River, or a hero that manages to dodge every round from a trio of machine guns. The least we can do is provide them with a believable background for our characters, and as I hope I've shown, the right job can go a very long way to do that.