Because if you were consuming 4000 calories of food and drink before and reduced it down to 3000 by drinking diet soda. It's still a lot, but less. Which is better.

Oh I'm just questioning why you think I would count my mom's calorie intake, or why it would specifically matter.

It specifically matters because the number influence on a person's weight is calorie intake/calorie usage.

Other things might influence to a degree, but the main problem is always going to be caloric in nature.