When you take notes of how you feel during paying for something, you start becoming more mindful of your actions, and more aware of what guides your buying decisions. Maybe you act impulsively during a purchase or try to compensate for boredom/ sadness/ gluttony?
Tracking expenses also helps you realize how your money is being spent, where do they go, what exactly do you buy. Over time you will also be able to spot your spending patterns and unhelpful behaviors. For example, you may notice that you buy a specific thing every time you feel upset (e.g., chocolate after an argument). Knowing what feelings and mental states drive your purchases will help you manage your overspending habits.
Observing other regularities, such as purchases at a specific time of the day or unpredictable expenses will also be useful when you get down to budgeting. Take note of all of the expenses, it doesn’t matter how small they may feel. These are the little things that add up to significant amounts which you later can’t recall where they went (spending leaks).