Care to share the link to your website?
If your site is not "static" (.htm files) the content is generated dynamically either from database or with the help of templates. Internal
SEO (as part of the overall
SEO) deals with web pages content (pages titles, text, etc.) they will need to see how the web pages are built. This can be the reason why they need access to the "raw" files ... or maybe they just wanna hijack your site, lol!
Just by visiting your website and looking at the web page source doesn't help as it displays the "finished product" ... they need to see how the page was constructed.