Use of Aura Handler in Lightning Component

<aura:component >
    <aura:attribute name='aVal' type='integer' required='true'/>
    <aura:attribute name='bVal' type='integer' required='true' />
    <aura:attribute name='addValue' type='integer' />
    <aura:attribute name='subValue' type='integer' />
    <aura:attribute name='mulValue' type='integer' />
    
    <aura:handler name='init' value='{!this}' action='{!c.addFunction}' />
    <aura:handler name='init' value='{!this}'  action='{!c.subFunction}' />
    
    <div>
        A Value = {!v.aVal} <br/>
        B Value = {!v.bVal} <br/>
        Addition= {!v.addValue} <br/>
        Multiplication = {!v.mulValue} <br/>
        Subtraction = {!v.subValue} 
    </div>
</aura:component>

({
	addFunction : function(component) {
		var a = component.get('v.aVal');
        var b = component.get('v.bVal');
        //Business logic
        var add = a+b;
        var mul = a*b;
        //setting value to attribute
        component.set('v.addValue', add);
        component.set('v.mulValue', mul);
	},
    
    subFunction : function(component) {
		var a = component.get('v.aVal');
        var b = component.get('v.bVal');
        //Business logic
        var sub = a-b;
        
        //setting value to attribute
        component.set('v.subValue', sub);
	}
})

<aura:application >
    <c:basicHandler aVal='20' bVal='10' />
</aura:application>