They are different to make life easy for you.
Suppose your sales page URL is: http;//originalsalespage.com
Then you want to change the sales page URL to: http;//newsalespage.com
Without this feature, you'd need to change this URL on ALL your web properties one by one by hand, which is very tiring and troublesome. With this feature, all you need to do is simply change the URL on 1 (ONE) place -- inside your Zaxaa account >> products >> edit the product in question >> page settings.
So, it's highly recommended that you use the sales page URL given to you under "Integration Settings" tab.
The same concept applies to your thank you page URL.