String.format() requires that you supply all the arguments corresponding to the format specifiers. Missing arguments usually leads to an improperly formatted string. Such strings can critically alter the behaviour and control-flow of the program if the program relies on it.
val intro = String.format("My name is %s and I am %d years old", name)
val intro = String.format("My name is %s and I am %d years old", name, age)