Why use symbols instead of string keys in this case?

So Jason creates the class "Monster", initializes it with an instance variable called "actions" that is a hash with the key "scream" and value 0.

class Monster 
    attr_reader :name, :actions

    def initialize(name)
        @name = name
        @actions = {
                scream: 0
        }
    end

    def say_something(&block)
        block.call
    end

    def scream(&block)
        actions[:scream] += 1
        print "#{name} screams!"
        yield()
    end

end

Now, in the method "scream", he has to access the instance variable "scream" as a symbol so he can increment the instance variable's value:

    def scream(&block)
        actions[:scream] += 1
        print "#{name} screams!"
        yield()
    end

Can someone clarify why he chose to create "scream" as a symbol instead of a string key/value pairing like:

 @actions = { "scream" => 0}

It is currently my understanding that you should use symbols when you want to have an immutable value in your object, correct? If so why use a symbol when we want to increment the value of scream when we call the method "scream"?