When you buy weapons and vehicles, you will see a value for upkeep. Upkeep is the amount of money that is taken away from your hourly income. So, if you were making $10,000 an hour and you bought a Vehicle with an Upkeep of $1,000, your hourly income would be $9,000.
Vehicles and weapons make your attack/defense scores higher, increasing your chances of winning against your opponents.