If you wanted to know all expenses for each individual vacation, tags would be VERY useful.
For example, you vacation to wonderful Kiln, MS. Create a tag called Kiln. Now you can tag whatever expenses (categories) you have associated with that trip: Fuel, ammunition, mosquito repellant, hardhat, Pepto Bismol, high boots, radar detector, etc. -- without re-creating these categories for, say, your next vacation to Bogalusa, LA.