So you know how much money you have.
There are many occupations in which addition is required. Accountancy strikes me as a major one, especially if you are required to perform quick, simple ones that would be more time consuming using a calculator to find out. People everyday paying for goods use it to make sure they were not given the wrong amount of change. People eating out use it to add 10% onto their amount to calculate the tip.
It just helps people in life generally who want to make quick calculations without the need for a calculator.
Most children will be working out how many days it is until Christmas now, and we adults always know how many days it is until payday because we all use basic addition to add up the days.
We need addition to work out when to take tablets in equal doses throughout the day, and also when to get a new prescription based on how many pills we have left.
If we have people over for dinner, we need to add up how much food we need to get.
At the gas station, we have to know how much gas costs and how much we want to spend as we need to work out how much we can afford.
All maths is used in daily life but probably the most common is addition, we would be lost without it.