Why use constructors

Question

Why use a constructor when you could just assign the variable a value when you initialize it?

Answer 1

Assigning the variable can be a default value.
But that's something ALL instances of the class will have.
With a constructor, you can change the values to be different for every object.

Here's an example

Lets say I have the constructor:

int age;
String name;

Person(int age, String name){
this.age = age;
this.name = name;
doStuff();
}

private void doStuff(){}

If I wrote this instead:
String name = "bob";
int age = 24;
Every object instance is going to have that age and name. So, I'd have to change it with a setter.

Meanwhile, I could have just changed it with the constructor instead..

Constructor:
Person bob = new Person(19, "Bob");
Person joe = new Person(88, "Joe");
Person lia = new Person(22, "Lia");
Person george = new Person(28, "George");

No Contructor:
Person joe = new Person();
/* keep in mind, this actually still uses the default contructor */
joe.setName("Joe");
joe.setAge(88);
joe.doStuff();

See how redundant / tedious that is?
For one person!
Plus, the doStuff() method is now forced to be public.
Far easier with contructors.

Answer 2

when you initiate and give initial values as you say, you're using a constructor. If you don't give initial values, guess what. Yep, you're using a constructor.
That's what constructors are for, if you don't create them, java uses default constructors.