Why do we need External dtd in blind XXE?

Finally, after doing a hard search I come to a point Parameter Entities are not allowed into Internal DTD subset i.e.

<!DOCTYPE r [ Not allow to set parameteried entity.] 

If I am wrong then clarify me. I read that on this XML doc.

Interaction with an external DTD is required so that you can use nested parameters. In an internal DTD the following is allowed: <!DOCTYPE alfa [ <!ENTITY bravo "charlie"> ]> However, in an internal DTD the following is not allowed: <!DOCTYPE alfa [ <!ENTITY bravo "charlie"> <!ENTITY delta SYSTEM "http://&bravo;.echo.com"> ]>

The link posted by @act1vand0 tells us this:

External DTD allows us to include one entity inside the second, but it is prohibited in the internal DTD. (https://mohemiv.com/all/exploiting-xxe-with-local-dtd-files/)

The PortSwigger documentation on blind XXE explains it in further detail:

The preceding technique works fine with an external DTD, but it won't normally work with an internal DTD that is fully specified within the DOCTYPE element. This is because the technique involves using an XML parameter entity within the definition of another parameter entity. Per the XML specification, this is permitted in external DTDs but not in internal DTDs. (Some parsers might tolerate it, but many do not.) (https://portswigger.net/web-security/xxe/blind)