Why to put val or var in kotlin class constructors

If you omit val or var in in a constructor, then the only places that can access these parameters are initialization statements that are evaluated at construction time. See https://kotlinlang.org/docs/reference/classes.html

This is useful when you want to do something with a value before storing it. In Java you would put that code a constructor body

class Person(firstName: String, lastName: String) {
    // directly in val / var declarations
    val firstName = firstName.capitalize()
    val lastName = lastName

    // in init blocks
    val fullName: String
    init {
        fullName = "$firstName $lastName"
    }

    // secondary constructors can only see their own parameters
    // and nothing else can access those
    constructor(fullName: String) : this("", fullName)
}

But it also works for delegation using by

interface Named {
    fun getName(): String
}
class Human(private val fname: String, private val lname: String) : Named {
    override fun getName() = "$fname + $lname" // functions need val since
                                               // value is resolved after construction
}
class Person2(firstName: String, lastName: String) : Named by Human(firstName, lastName)

class Person3(human: Human) : Named by human {
    constructor(firstName: String, lastName: String): this(Human(firstName, lastName))
}

Or in property delegation

class Person4(firstName: String, lastName: String) {
    val fullName: String by lazy { "$firstName $lastName" }
}

Note: the closure is captured at initialization time, the values are therefore still available when lazy evaluates eventually.

If val or var is omitted then they won't be properties, but parameters passed to constructor. You won't be able to work with them outside of constructor.