Autoresponder vs Thank you page: what is the use of those two features?

Please explain why I would use autoresponder to inform submitters that I received their form when I have a thank you page.  Doesn't the thank you page do the same thing?  I don't understand the difference.

The thank you page is just to let the user know that the form was submitted successfully, but what if the user would like to have a copy of what he submitted? That's the purpose of the autoresponder, but if your users are not asking for that, simply delete the autoresponder: https://www.jotform.com/help/132-How-to-Delete-Notifications-Autoresponder 

Thanks for making the purpose of the autoresponder clear.  I would not have figured out that the purpose was to let people get an email with what they submitted.  Given under "Email Subject 'change the subject title of your email content'" your pre-populated box says "We have received your response for {form_title} {yourName}.  This what confused to me because it clearly states it is informing people their form was received - just like the thank you page.  Or am I still not understanding?

The {form_title} and {yourName} are variables, the ones that get the values from your form fields. So, you can remove them. But let's say you want to put a more customized title, something like:

"Thank you "name of the person who submitted the form", we have received your submission"

You would need to set your title as "Thank you {yourName}, we have received your submission", so the submitter will see it like "Thank you BDAVID, we have received your submission".

So, each individual who submits your form, will receive the notification with his name in the title.

You can put any information from your form fields in your title if you want to, simply click on the pencil icon and add the field you want to:

Let us know if you need more help.