When first starting a budget, we have great intentions of classifying each one of our purchases, cutting back where we need to and, ultimately, pocketing a few extra dollars each month in the form of savings. The problem is that there always seems to be that "weird" expense that comes up each month or week that you KNOW you won't incur again.
But then, each month, you keep encountering more of them: a laptop for school, the ball game your friend invited you to at the last minute, a doctors visit that cost $100 out-of-pocket. Start treating these as inevitable costs of living rather than one-offs that topple your budget.
At least, with a catch-all category, you deliberately account for the unplanned.