Most travel budgets fail not because of one big overspend, but because of a dozen small categories that were never accounted for in the first place. A budget built only around flights and hotels usually leaves out food, local transport, activities, and the inevitable one-off purchase that shows up mid-trip.
Flights, hotels, and any pre-booked tours are fixed and easy to total up front. The harder part is estimating daily spending - food, transit, small purchases - which is where most budgets quietly break down. Researching a realistic daily average for the destination, rather than guessing, makes the whole plan far more reliable.
Adding 10-15% on top of the total estimate isn't pessimism - it's accounting for the exchange rate moving, an unplanned activity, or a delay that requires an extra night somewhere. A budget with no buffer at all tends to get abandoned by day three.
Tracking spending daily, even loosely, catches a budget drifting off course while there's still time to adjust - rather than discovering the gap only after coming home.