The calorie surplus and deficit is a two part equation.
Part 1 - Calories in. This is the food that you eat.
Part 2 - Calories out. This is the calories burned throughout the day. This includes the Basal Metabolic Rate (BMR), Thermic Effect of Food (TEF), Non-exercise Activity Thermogenesis (NEAT) , and exercise. All that combined is called the Total Daily Energy Expenditure (TDEE).
If the calories in is greater than the calories out, then you're in a surplus. If the calories in is less than the calories out, you are in a deficit.
if I eat less calories, what's the need of cardio then?
Speaking strictly in terms of weight management, it can make dieting easier. Cardio increases the "energy out" part of the equation. This gives a bit of a buffer for the "energy in" part so people can eat a little bit more. It also allows for compensation on mistakes. It also gives you something to do so you're not thinking about food.
However, it is possible to overdo it. An extreme amount of cardio tends to make people feel hungrier throughout the day, so there is an increased risk of binging.
It can also lower the NEAT portion of the equation because you become tired. For example, you may wake up in the morning and bike for two hours and end up burning 800 calories, but now you're too tired or in pain to clean the house. So instead of being active throughout the day you end up lounging. You may end up in a deficit at the end of the day, but it's lower than expected.
So it's important to find a balance. Cardio for weight loss shouldn't be exhausting (unless you find that motivating). How intense it should be is entirely based on your fitness level. If you feel too exhausted to go about your normal day then you probably did too much.
Does cardio enable some mechanism and help us target fat ?
There is the so called "fat burning zone" that people often strive to reach in cardio in which the body burns more fat than carbohydrate or protein for energy. However, in the grand scheme of things, the effect is minimal.
For one, the amount of calories burned during cardio just isn't that much. Unless you spend hours moving and a very hard pace, then the most you probably burn is 100-400 calories per session.
Second, if you burn mostly fat during a cardio session, then carbohydrate gets used a bit more for the rest of the day. If you burn mostly carbs during a session, then fat oxidation increases a tiny bit throughout the rest of the day. It balances out in the end.