what is the use of <aura:set> in Salesforce lightning ?

Use <aura:set> in the markup of a sub component to set the value of an inherited attribute.

Let's look at an example. Here is the c:setTagSuper component.

<!--c:setTagSuper-->
<aura:component extensible="true">
<aura:attribute name="address1" type="String" />
setTagSuper address1: {!v.address1}<br/>
</aura:component>

c:setTagSuper outputs:

setTagSuper address1:

The address1 attribute doesn't output any value yet as it hasn't been set.

Here is the c:setTagSub component that extends c:setTagSuper.

<!--c:setTagSub-->
<aura:component extends="c:setTagSuper">
<aura:set attribute="address1" value="808 State St" />
</aura:component>

c:setTagSub outputs:

setTagSuper address1: 808 State St