Sometimes it's important - to the plot, I mean - what someone does for a living, sometimes it isn't... yet people self-identify with 'what they do' to such an extent that if an NPC cannot tell you instantly what he does for a living he becomes suspect automatically. As a GM, you know which NPCs are suspicious characters and which ones are just regular joes who happen to be in the vicinity... but can you rattle off an assortment of occupations to cover all the people in the scene as your players prowl around asking them questions?
Thought not. Grab this, rattle some dice, and out come sensible trades that a random assortment of passers-by might well practise. If you are planning an adventure, rather than being caught on the hop whilst running it, do the same and keep your creative juices for important stuff. Even players might find it handy... you know you're really a monster hunter, say, and that is what the game is about, but there probably isn't much money in it so a job to pay the rent is a good idea... so what do you do? Some just pick a trade they know or one that sounds vaguely interesting, but using a list like this can spawn a whole wealth of ideas to round out your character. So he monster-hunts in the game, but if he's a chef he'll bring different background skills than if he's a HVAC mechanic, he'll have different resources if he's a gun shop owner than if he's a tailor or a subway worker...
Bring a breath of reality into your game... then go hunt monsters anyway. It's more fun even if it doesn't pay too well.