what is the purpose of {{block.super}} in django template?

Sure. So lets create a block in example.html like so:

{% block content %}
    <p>Some example text.</p>
{% endblock %}

And then we can extend that block in a second template:

{% extends 'example.html' %}

{% block content %}
    <p>The previous example text is replaced by this text.</p>
{% endblock %}

In the example above, only the latter sentence would show up. "Some example text." is being overridden and replaced by "The previous example text is replaced by this text.". You would use {{ block.super }} if you want both things to show up:

{% extends 'example.html' %}

{% block content %}
    {{ block.super }}
    <p>The previous example text is replaced by this text.</p>
{% endblock %}

So with the call to super, you can render whatever was declared in the first template rather than replacing it whatever you put in the template that extends it.

From the Django documention: "If you need to get the content of the block from the parent template, the {{ block.super }} variable will do the trick. This is useful if you want to add to the contents of a parent block instead of completely overriding it. Data inserted using {{ block.super }} will not be automatically escaped..."